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.

plugin/skills/reusing-code/SKILL.md

@@ -20,6 +20,14 @@ through recall/auto-inject; this skill is the active reflex around that.
   `list_snippets` searches every project by default; that's deliberate, since a
   helper you need here was quite possibly written somewhere else. Narrow with
   `project_id` only when you specifically want this project's own.
+- **About to edit an existing file? Ask by place, not just by meaning.**
+  `list_snippets(repo="…", path="…")` answers "what canonical helpers are already
+  recorded here?" — `path` matches the exact file or anything beneath it, so
+  `path="frontend/src"` covers the whole tree. Cheaper and sharper than a
+  wording search when you already know where the code is going, and it catches
+  the helper you'd otherwise duplicate a few lines down. `symbol="…"` narrows
+  further, and all three must match the same recorded location. Combine with `q`
+  to ask both at once.
 - If a snippet fits, pull it in full with `get_snippet(id)` and reuse it — its
   `location` points at the reference implementation. Adapt, don't re-derive.
 - If auto-inject already surfaced a snippet title that looks relevant, that's