feat(write-path): the derive arm — the hint names a duplicate family (no canon) or a canon elsewhere for the shapes being written; exclude_derive channel (#2900, milestone 299 step 2)

shape_ledger.write_time_derive asks the ledger what it knows about each
named (kind, symbol): a derive-grouped family (identical body / same name
in N other files) -> "derive it now, do not add a copy"; a canonical row
at another path -> "canon #N at <path>, reuse". Judged rows at the path and
the canon own file stay silent. Rendered by _derive_line beside the
divergence line; keyed (group id / canon:<id>) on a third per-session dedup
channel in the hook (.derive.ids -> exclude_derive=).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/services/plugin_context.py

@@ -706,6 +706,7 @@ async def build_write_path_hint(
     exclude_sync_ids: list[int] | None = None,
     stamp_shapes: list[tuple[str, str]] | None = None,
     repo_key: str = "",
+    exclude_derive: list[str] | None = None,
 ) -> dict:
     """Prior-art hint for the plugin's PreToolUse hook on Write/Edit.
 
@@ -765,7 +766,7 @@ async def build_write_path_hint(
     """
     cfg = await get_writepath_config(user_id)
     empty = {"context": "", "note_ids": [], "sync_note_ids": [], "config": cfg,
-             "stamped": [], "divergence": []}
+             "stamped": [], "divergence": [], "derive": [], "derive_keys": []}
     path = (path or "").strip()
     if not cfg["enabled"] or not path:
         return empty
@@ -935,7 +936,20 @@ async def build_write_path_hint(
             )
         except Exception:
             logger.warning("write-time divergence check failed", exc_info=True)
-    if not synced and not menu and not stamped and not divergence:
+    # The in-band DERIVE check (#2900): the ledger's own knowledge of the
+    # names being written — a duplicate family with no canon, or a canon
+    # recorded elsewhere. This is the arm the by-name local grep could not
+    # be: it knows whether the other copies are canon or stray. Keyed per
+    # session (`exclude_derive`) so a family is named once, not per edit.
+    derive: list[dict] = []
+    if stamp_shapes and project_id:
+        try:
+            found = await shape_ledger_svc.write_time_derive(project_id, path, stamp_shapes)
+            skip = set(exclude_derive or [])
+            derive = [d for d in found if d.get("key") not in skip]
+        except Exception:
+            logger.warning("write-time derive check failed", exc_info=True)
+    if not synced and not menu and not stamped and not divergence and not derive:
         return empty
 
     owners = await owner_names_for({
@@ -1003,6 +1017,8 @@ async def build_write_path_hint(
         lines.append(_stamp_line(path, stamped))
     if divergence:
         lines.append(_divergence_line(path, divergence))
+    if derive:
+        lines.append(_derive_line(path, derive))
 
     # 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
@@ -1027,9 +1043,40 @@ async def build_write_path_hint(
         "config": cfg,
         "stamped": stamped,
         "divergence": divergence,
+        "derive": derive,
+        "derive_keys": [d["key"] for d in derive],
     }
 
 
+def _derive_line(path: str, derive: list[dict]) -> str:
+    """The ledger's word on the names being written (#2900): a duplicate
+    family to derive, or a canon to reuse — said at the write."""
+    parts = []
+    for d in derive:
+        if d.get("canon"):
+            c = d["canon"]
+            parts.append(
+                f"`{c['label']}` is canon — snippet #{c['snippet_id']} at `{c['path']}`; "
+                "pull it and reuse, don't redefine"
+            )
+            continue
+        f = d["family"]
+        how = "identical body" if f.get("identical") else "same name defined"
+        files = ", ".join(f"`{x}`" for x in f.get("files") or [])
+        more = f.get("file_count", 0) - len(f.get("files") or [])
+        if more > 0:
+            files += f" +{more} more"
+        parts.append(
+            f"`{f['label']}` is a duplicate family with no canon — {how} in "
+            f"{f.get('file_count', 0)} other file(s): {files}; derive it now: "
+            "record the canon (create_snippet) and make the copies instances "
+            "(classify_shapes) — or, if these are convention not copies, "
+            "`classify_shapes(..., status=\"exempt\", reason_code=\"convention-plumbing\")` "
+            "dismisses the family — rather than adding another copy"
+        )
+    return f"> Shape ledger at `{path}`: " + "; ".join(parts) + "."
+
+
 def _divergence_line(path: str, divergence: list[dict]) -> str:
     """Button B where button A is canon — named at the write (#2793)."""
     parts = [