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}` : ""}`);
 }

frontend/src/views/SnippetListView.vue

@@ -13,6 +13,27 @@ const error = ref<string | null>(null);
 const search = ref("");
 let searchTimer: ReturnType<typeof setTimeout> | undefined;
 
+// Reverse lookup — "what already lives here?" All three must match the same
+// recorded location; path also matches anything beneath it.
+const showLocationFilter = ref(false);
+const locRepo = ref("");
+const locPath = ref("");
+const locSymbol = ref("");
+const locationActive = computed(
+  () => !!(locRepo.value.trim() || locPath.value.trim() || locSymbol.value.trim()),
+);
+function clearLocation() {
+  locRepo.value = "";
+  locPath.value = "";
+  locSymbol.value = "";
+  loadSnippets();
+}
+function toggleLocationFilter() {
+  showLocationFilter.value = !showLocationFilter.value;
+  // Collapsing while filtered would hide why the list is short.
+  if (!showLocationFilter.value && locationActive.value) clearLocation();
+}
+
 // Multi-select → merge
 const selectMode = ref(false);
 const selectedIds = ref<Set<number>>(new Set());
@@ -70,7 +91,12 @@ async function loadSnippets() {
   loading.value = true;
   error.value = null;
   try {
-    const data = await listSnippets({ q: search.value.trim() || undefined });
+    const data = await listSnippets({
+      q: search.value.trim() || undefined,
+      repo: locRepo.value.trim() || undefined,
+      path: locPath.value.trim() || undefined,
+      symbol: locSymbol.value.trim() || undefined,
+    });
     snippets.value = data.snippets;
   } catch {
     error.value = "Couldn't load your snippets.";
@@ -84,6 +110,9 @@ function onSearchInput() {
   clearTimeout(searchTimer);
   searchTimer = setTimeout(loadSnippets, 300);
 }
+// Same debounce for the location fields — typing a path shouldn't fire a query
+// per keystroke.
+const onLocationInput = onSearchInput;
 
 onMounted(loadSnippets);
 
@@ -127,6 +156,48 @@ function languageOf(tags: string[]): string {
         @input="onSearchInput"
         @keydown.enter="loadSnippets"
       />
+      <button
+        class="btn-ghost"
+        :class="{ 'filter-on': locationActive }"
+        :aria-expanded="showLocationFilter"
+        aria-controls="location-filter"
+        @click="toggleLocationFilter"
+      >
+        <!-- Say it in the label, not only in the accent — the state has to reach
+             a screen reader too. -->
+        {{ locationActive ? "Location · filtering" : "Location" }}
+      </button>
+    </div>
+
+    <!-- Reverse lookup: what's already kept in this repo / file / symbol. -->
+    <div v-if="showLocationFilter" id="location-filter" class="location-row">
+      <input
+        v-model="locRepo"
+        class="loc-input"
+        placeholder="repo"
+        aria-label="Filter by repo"
+        @input="onLocationInput"
+        @keydown.enter="loadSnippets"
+      />
+      <input
+        v-model="locPath"
+        class="loc-input loc-input-wide"
+        placeholder="path — matches the file or anything under it"
+        aria-label="Filter by path"
+        @input="onLocationInput"
+        @keydown.enter="loadSnippets"
+      />
+      <input
+        v-model="locSymbol"
+        class="loc-input"
+        placeholder="symbol"
+        aria-label="Filter by symbol"
+        @input="onLocationInput"
+        @keydown.enter="loadSnippets"
+      />
+      <button v-if="locationActive" class="loc-clear" @click="clearLocation">
+        Clear
+      </button>
     </div>
 
     <div v-if="loading" class="skeleton-grid">
@@ -138,15 +209,24 @@ function languageOf(tags: string[]): string {
     <div v-else-if="snippets.length === 0" class="empty-state-rich">
       <div class="empty-icon">❭_</div>
       <p class="empty-title">
-        {{ search.trim() ? "No snippets match your search" : "No snippets kept yet" }}
+        {{ locationActive
+          ? "Nothing kept at that location yet"
+          : search.trim()
+            ? "No snippets match your search"
+            : "No snippets kept yet" }}
       </p>
       <p class="empty-sub">
-        {{ search.trim()
-          ? "Try a different term, or clear the search."
-          : "Record a reusable function or component and it will be offered back to you later." }}
+        {{ locationActive
+          ? "No recorded snippet lives there — so whatever you're about to write is new. Widen the path, or clear the filter."
+          : search.trim()
+            ? "Try a different term, or clear the search."
+            : "Record a reusable function or component and it will be offered back to you later." }}
       </p>
+      <button v-if="locationActive" class="empty-action" @click="clearLocation">
+        Clear location filter
+      </button>
       <button
-        v-if="!search.trim()"
+        v-else-if="!search.trim()"
         class="empty-action"
         @click="router.push('/snippets/new')"
       >
@@ -278,6 +358,62 @@ function languageOf(tags: string[]): string {
 
 .search-row {
   margin-bottom: 1.25rem;
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+  flex-wrap: wrap;
+}
+/* Filter is engaged — the accent marks "you are here", per the design system. */
+.filter-on {
+  border-color: var(--color-primary);
+  color: var(--color-primary);
+}
+
+.location-row {
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+  flex-wrap: wrap;
+  margin: -0.5rem 0 1.25rem;
+}
+.loc-input {
+  flex: 1 1 8rem;
+  min-width: 0;
+  max-width: 12rem;
+  padding: 0.4rem 0.65rem;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  background: var(--color-bg);
+  color: var(--color-text);
+  font-size: 0.85rem;
+  font-family: var(--font-mono, ui-monospace, "JetBrains Mono", monospace);
+  box-sizing: border-box;
+}
+.loc-input-wide {
+  flex: 2 1 16rem;
+  max-width: 26rem;
+}
+.loc-input::placeholder {
+  font-family: inherit;
+  color: var(--color-text-muted);
+}
+.loc-input:focus {
+  outline: none;
+  border-color: var(--color-primary);
+  box-shadow: var(--focus-ring);
+}
+.loc-clear {
+  padding: 0.4rem 0.65rem;
+  border: none;
+  background: transparent;
+  color: var(--color-text-secondary);
+  font-size: 0.85rem;
+  font-family: inherit;
+  cursor: pointer;
+  text-decoration: underline;
+}
+.loc-clear:hover {
+  color: var(--color-text);
 }
 .search-input {
   width: 100%;