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.

frontend/src/api/snippets.ts

@@ -80,13 +80,26 @@ export interface SnippetInput {
   force?: boolean;
 }
 
+/** `repo`/`path`/`symbol` are the reverse lookup — "what already lives here?"
+ *  They must all match the same recorded location; `path` also matches anything
+ *  beneath it. Combinable with `q`. */
 export async function listSnippets(
-  params: { q?: string; tag?: string; projectId?: number | null } = {},
+  params: {
+    q?: string;
+    tag?: string;
+    projectId?: number | null;
+    repo?: string;
+    path?: string;
+    symbol?: string;
+  } = {},
 ): Promise<{ snippets: SnippetListItem[]; total: number }> {
   const qs = new URLSearchParams();
   if (params.q) qs.set("q", params.q);
   if (params.tag) qs.set("tag", params.tag);
   if (params.projectId) qs.set("project_id", String(params.projectId));
+  if (params.repo) qs.set("repo", params.repo);
+  if (params.path) qs.set("path", params.path);
+  if (params.symbol) qs.set("symbol", params.symbol);
   const query = qs.toString();
   return apiGet(`/api/snippets${query ? `?${query}` : ""}`);
 }