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.

tests/test_routes_snippets.py

@@ -72,6 +72,19 @@ def test_project_scoping_reaches_every_caller():
     assert "project_id" in inspect.getsource(routes.list_snippets_route)
 
 
+def test_location_lookup_reaches_every_caller():
+    """The reverse lookup — "what already lives here?" — has to be askable from
+    both surfaces, or an agent and a human get different answers about the same
+    file (rule #33)."""
+    from scribe.mcp.tools import snippets as tools
+    from scribe.routes import snippets as routes
+    from scribe.services import snippets as svc
+    for key in ("repo", "path", "symbol"):
+        assert key in inspect.signature(svc.list_snippets).parameters
+        assert key in inspect.signature(tools.list_snippets).parameters
+        assert f'request.args.get("{key}"' in inspect.getsource(routes)
+
+
 def test_update_field_map_matches_service_kwargs():
     """Every field the PATCH route forwards must be a real update_snippet kwarg
     (rule #33 interface-contract parity)."""