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/routes/snippets.py

@@ -56,9 +56,15 @@ async def list_snippets_route():
         project_id = int(request.args.get("project_id", 0) or 0) or None
     except (TypeError, ValueError):
         project_id = None
+    # Reverse lookup — "what already lives here?" Empty args are ignored, so the
+    # plain list is unchanged.
+    repo = request.args.get("repo", "")
+    path = request.args.get("path", "")
+    symbol = request.args.get("symbol", "")
     limit, offset = parse_pagination()
     items, total = await snippets_svc.list_snippets(
         uid, q=q, tag=tag, limit=limit, offset=offset, project_id=project_id,
+        repo=repo, path=path, symbol=symbol,
     )
     # Mark rows owned by someone else so the UI can show whose they are — an
     # unmarked row in your own list reads as one you recorded and vetted.