feat(snippets): reverse lookup — find snippets by repo/path/symbol

"What canonical helpers already live in this file?" was unanswerable:
location lived only in the body markdown. It is now a jsonpath containment
query over the `notes.data` mirror added by migration 0070.

- One predicate in two dialects in services/knowledge.py: SQL (`data @?`,
  applied in the browse arm and the keyword arm before count/pagination, so
  totals stay honest) and Python (`location_matches`, for the semantic arm
  which post-filters candidates it already holds). Both must change together.
- Parts are ANDed within a SINGLE locations entry — repo A in one entry and
  path B in another is not "recorded at A/B". `path` also matches as a
  directory prefix, via jsonpath `starts with` rather than `@>`, which the
  same GIN index serves.
- `repo`/`path`/`symbol` reach the service, the REST list and the MCP tool
  under one name with one default (rule #33); the MCP docstring teaches the
  place form, and so does the reusing-code skill (plugin.json bumped).
- UI: a Location disclosure beside the snippet search, with its own empty
  state — "nothing kept there, so what you're about to write is new."

Settles #2083's open question (pre-0070 NULL `data`) by backfilling after
all: `backfill_snippet_data` runs at startup, deriving the mirror from the
body with the same parser the read path trusts. 0070's caution was about
mangling a hand-edited body; this never touches the body. The alternative
was a permanent second body-regex arm, or a query that silently answers
"nothing here" for an old snippet and gets the helper written twice.

Refs #2083, milestone #232.

src/scribe/mcp/tools/snippets.py

@@ -19,9 +19,16 @@ from scribe.services import systems as systems_svc
 
 async def list_snippets(
     q: str = "", tag: str = "", limit: int = 50, project_id: int = 0,
+    repo: str = "", path: str = "", symbol: str = "",
 ) -> dict:
     """List recorded snippets (reusable functions/components).
 
+    Two ways to ask, usable together: by MEANING (`q` — "what do I need this code
+    to do?") and by PLACE (`repo`/`path`/`symbol` — "what canonical helpers
+    already live in the file I'm about to edit?"). Reach for the place form
+    before you write or change code in a file: it is the cheap way to find prior
+    art you'd otherwise duplicate a few lines down.
+
     Args:
         q: Free-text search across name + body (optional). Matches on meaning as
             well as wording, so describe what you need the code to DO.
@@ -30,6 +37,16 @@ async def list_snippets(
         project_id: Narrow to one project. 0 (default) searches every project —
             usually what you want, since a helper you need here may well have
             been written somewhere else.
+        repo: Narrow to snippets recorded in this repo, matched exactly — the
+            same repo string used when recording, e.g. "Scribe".
+        path: Narrow to snippets recorded at this path. Matches the exact file
+            OR anything beneath it, so "frontend/src" finds
+            "frontend/src/lib/x.ts" as well as itself.
+        symbol: Narrow to snippets recorded under this symbol name, exactly.
+
+    `repo`/`path`/`symbol` must all match the SAME recorded location, so a
+    snippet that lives in repo A and, separately, at path B in another repo is
+    not returned for repo=A + path=B.
 
     Returns {"snippets": [{id, title, tags, preview}], "total": int}. The title
     reads "name — when to reach for it"; open one in full with get_snippet(id).
@@ -44,6 +61,7 @@ async def list_snippets(
     items, total = await snippets_svc.list_snippets(
         uid, q=q or None, tag=tag, limit=max(1, min(limit, 100)),
         project_id=project_id or None,
+        repo=repo, path=path, symbol=symbol,
     )
     return {
         "snippets": await access_svc.label_shared_items(uid, items),