fix(write-path): disclose cross-language prior art instead of hiding it

Closes #2244. Retrieval matches on concept, and concepts are language-agnostic:
asking about a TypeScript union-find scores 0.72-0.73 against a PYTHON snippet,
comfortably over the 0.68 bar. That is useful — a different-language solution
gives you the shape even when the code isn't reusable — but the menu line said
nothing about it, so the reader either dismissed a good structural reference or
pasted Python into a .ts file.

Worth noting this predates the concept-query change: raw TS code already matched
the Python snippet at 0.73, because the embedder reads identifiers and structure
semantically rather than syntactically. The fail state has been shipping quietly;
#2242 only made it an intended use rather than an accident.

- knowledge._note_to_item projects `language` from the data mirror, same shape as
  the existing verification projection — a plain column read, no body parsing.
- The semantic arm carries language through on the item it builds; it is the arm
  where these arise, since a snippet recorded AT the path you're editing is
  almost never in another language.
- _prior_art_line folds it into the marker: [similar 0.72 · python]. Together
  with the score rather than after the title, because the two jointly are the
  judgement being offered.
- One explanatory line is added to the menu, and only when something on it is
  actually tagged.

Two deliberate calls:

LABEL, DON'T FILTER. A stricter threshold for foreign-language hits would
suppress exactly the shape-borrowing this exists for. They were never the
problem; their being undisclosed was.

ONLY CLAIM A MISMATCH YOU CAN ESTABLISH. _foreign_language returns "" when either
side is unknown — unrecognised extension, or a snippet with no recorded language.
A wrong "· python" is worse than no tag. Same-language hits stay unlabelled, so
the common case keeps a clean line and the preamble stays off the menu entirely.
Operator-typed language names fold through an alias table first (py/python3 →
python, tsx → typescript, c++ → cpp); unrecognised names pass through lowercased,
which still makes an unknown-but-equal pair compare equal.

Trap found while building: _note() in the tests is a MagicMock, so `note.data`
auto-created a truthy mock that would have rendered its repr into a menu line.
Both test helpers now set data = None explicitly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

src/scribe/services/knowledge.py

@@ -211,6 +211,15 @@ def _note_to_item(note: Note) -> dict:
     # body parsing and stays a plain projection of the column. Omitted entirely
     # when unchecked, so "no key" and "never verified" don't become two states
     # the client has to tell apart.
+    # Snippet language, same reasoning as the verdict below: a plain projection of
+    # the `data` mirror, no body parsing. Needed because a prior-art hit in a
+    # DIFFERENT language than the file being written is useful as the shape of a
+    # solution but must not be mistaken for code to paste (#2244) — and the caller
+    # can only say "different" if the language is on the item.
+    language = (note.data or {}).get("language") if note.data else None
+    if language:
+        item["language"] = language
+
     verdict = (note.data or {}).get("verification") if note.data else None
     if verdict and verdict.get("status"):
         item["verification"] = {

src/scribe/services/plugin_context.py

@@ -391,15 +391,98 @@ async def build_autoinject_hint(
 # this exact file" is a stronger claim than "this resembles something".
 
 
-def _prior_art_line(item: dict, marker: str, owner: str | None) -> str:
-    """One menu line: `- #12 [here] "title"`, attributed when it isn't yours."""
+def _prior_art_line(item: dict, marker: str, owner: str | None, foreign_lang: str = "") -> str:
+    """One menu line: `- #12 [here] "title"`, attributed when it isn't yours.
+
+    A foreign language is folded into the marker (`[similar 0.72 · python]`)
+    rather than appended after the title, so the reader sees it while still
+    reading the score — the two together are the judgement being offered.
+    """
     title = (item.get("title") or "(untitled)").replace("\n", " ").strip()
-    line = f"> - #{item['id']} [{marker}] \"{title}\""
+    mark = f"{marker} · {foreign_lang}" if foreign_lang else marker
+    line = f"> - #{item['id']} [{mark}] \"{title}\""
     if owner:
         line += f" — shared by {owner}, treat as a suggestion"
     return line
 
 
+# --- cross-language prior art (#2244) ----------------------------------------
+# Retrieval is concept-shaped now, and concepts are language-agnostic: a query
+# about a TypeScript union-find matches a PYTHON snippet at 0.72-0.73, comfortably
+# over the bar. That is a feature — the operator's framing is "borrow the shape of
+# the solution even when the code isn't directly reusable" — but only if the line
+# SAYS so. An unlabelled Python hit offered while writing TypeScript either gets
+# dismissed as irrelevant or, worse, pasted into a .ts file. Measured note: this
+# cross-language matching predates concept queries; it was always happening, just
+# never disclosed.
+#
+# Deliberately NOT gated behind a stricter threshold for foreign-language hits: a
+# higher bar would suppress exactly the shape-borrowing this is for. Label, don't
+# filter.
+_LANG_BY_EXT = {
+    "py": "python", "pyi": "python",
+    "ts": "typescript", "tsx": "typescript", "mts": "typescript", "cts": "typescript",
+    "js": "javascript", "jsx": "javascript", "mjs": "javascript", "cjs": "javascript",
+    "vue": "vue", "svelte": "svelte",
+    "go": "go", "rs": "rust", "rb": "ruby", "php": "php",
+    "java": "java", "kt": "kotlin", "kts": "kotlin", "scala": "scala",
+    "c": "c", "h": "c", "cc": "cpp", "cpp": "cpp", "cxx": "cpp", "hpp": "cpp",
+    "cs": "csharp", "swift": "swift", "m": "objectivec", "mm": "objectivec",
+    "sh": "shell", "bash": "shell", "zsh": "shell", "fish": "shell",
+    "sql": "sql", "css": "css", "scss": "scss", "less": "less",
+    "html": "html", "htm": "html", "yml": "yaml", "yaml": "yaml",
+    "toml": "toml", "ini": "ini", "dockerfile": "dockerfile",
+    "ex": "elixir", "exs": "elixir", "erl": "erlang", "hs": "haskell",
+    "lua": "lua", "pl": "perl", "r": "r", "dart": "dart", "zig": "zig",
+}
+
+# `language` on a snippet is operator-typed free text, so fold the spellings that
+# mean the same thing before comparing. Anything unrecognised passes through
+# lowercased — an unknown-but-equal pair still compares equal, which is the only
+# thing this needs to get right.
+_LANG_ALIASES = {
+    "py": "python", "python3": "python",
+    "ts": "typescript", "tsx": "typescript",
+    "js": "javascript", "jsx": "javascript", "node": "javascript",
+    "sh": "shell", "bash": "shell", "zsh": "shell", "shell-script": "shell",
+    "c++": "cpp", "cplusplus": "cpp", "c#": "csharp", "objective-c": "objectivec",
+    "golang": "go", "rs": "rust", "rb": "ruby", "yml": "yaml",
+    "postgres": "sql", "postgresql": "sql", "psql": "sql",
+    "vuejs": "vue", "vue3": "vue",
+}
+
+
+def _canonical_language(name: str) -> str:
+    """Fold a free-text language name to a comparable token ("" if absent)."""
+    token = (name or "").strip().lower()
+    return _LANG_ALIASES.get(token, token)
+
+
+def _language_for_path(path: str) -> str:
+    """The language implied by a file path's extension ("" when unknown)."""
+    tail = (path or "").rsplit("/", 1)[-1].lower()
+    if tail.startswith("dockerfile"):
+        return "dockerfile"
+    if "." not in tail:
+        return ""
+    return _LANG_BY_EXT.get(tail.rsplit(".", 1)[-1], "")
+
+
+def _foreign_language(item: dict, target: str) -> str:
+    """The item's language when it DIFFERS from the target file's, else "".
+
+    Returns "" whenever either side is unknown: we can only claim a mismatch we
+    can actually establish, and a wrong "· python" tag is worse than no tag.
+    Same-language hits stay unlabelled so the common case keeps a clean line.
+    """
+    if not target:
+        return ""
+    theirs = _canonical_language(item.get("language") or "")
+    if not theirs or theirs == target:
+        return ""
+    return theirs
+
+
 def _concept_doc(code: str) -> str:
     """The first doc-ish prose in `code`: docstring, else JSDoc, else leading comments."""
     m = _CONCEPT_PY_DOC.search(code)
@@ -619,7 +702,14 @@ async def build_write_path_hint(
                     continue
                 scored.append((
                     f"similar {score:.2f}",
-                    {"id": int(note.id), "title": note.title, "user_id": note.user_id},
+                    {
+                        "id": int(note.id), "title": note.title, "user_id": note.user_id,
+                        # Carried so the line can disclose a cross-language hit
+                        # (#2244). The semantic arm is where these actually arise —
+                        # a snippet recorded at the path you're editing is almost
+                        # never in another language, but a concept match easily is.
+                        "language": (note.data or {}).get("language") if note.data else None,
+                    },
                 ))
 
     menu = (placed + scored)[:top_k]
@@ -631,19 +721,36 @@ async def build_write_path_hint(
         if it.get("user_id") is not None and int(it["user_id"]) != user_id
     })
 
+    target_lang = _language_for_path(path)
+    rendered: list[tuple[dict, str, str | None, str]] = []
+    for marker, item in menu:
+        owner_id = item.get("user_id")
+        owner = None
+        if owner_id is not None and int(owner_id) != user_id:
+            owner = owners.get(int(owner_id)) or "another user"
+        rendered.append((item, marker, owner, _foreign_language(item, target_lang)))
+
     lines = [
         f"> Prior art already recorded in Scribe for `{path}` — open one with "
         "`get_snippet(id)` and reuse it rather than writing a fresh one-off "
         "(titles only; shown once per session):",
     ]
+    # Say what a language tag MEANS, and only when one is actually on the menu.
+    # Without this the reader has to infer why "· python" is attached to a hit on
+    # a .ts file, and the two ways of guessing wrong are both bad: dismiss it as
+    # irrelevant, or paste Python into TypeScript. Retrieval matches on concept,
+    # so these are genuinely useful — as the SHAPE of a solution, not as code.
+    if any(lang for _i, _m, _o, lang in rendered):
+        lines.append(
+            "> A tagged language means that snippet is in a DIFFERENT language "
+            "than this file — it matched on what it does, so treat it as the "
+            "shape of a solution to adapt, not code to copy."
+        )
+
     note_ids: list[int] = []
-    for marker, item in menu:
+    for item, marker, owner, foreign_lang in rendered:
         note_ids.append(int(item["id"]))
-        owner_id = item.get("user_id")
-        owner = None
-        if owner_id is not None and int(owner_id) != user_id:
-            owner = owners.get(int(owner_id)) or "another user"
-        lines.append(_prior_art_line(item, marker, owner))
+        lines.append(_prior_art_line(item, marker, owner, foreign_lang))
 
     # Split by arm, which is the whole reason this table exists. The place arm
     # carries no score and so has no home in retrieval_logs; before #2085 a