feat(snippets): drift check — verify a snippet still matches its source

A recorded snippet points at a repo · path · symbol that WILL rot: files
move, symbols get renamed, implementations diverge from the copy stored
here. Nothing detected any of it, so a record degraded silently from
"canonical reference" to "confidently wrong" — worse than no record, since
it is surfaced with the same authority either way.

WHERE THE CHECK RUNS. Agent-side, which the task flagged as the design
question to settle first. Scribe has no checkout of the operator's repos
and must not acquire one: giving the server repo access would make every
install a credential problem and break instance-agnosticism (rule #115).
The agent already has the working tree, so it does the comparing; the
server remembers the verdict, makes it queryable, and knows when it has
expired. New MCP tool verify_snippet teaches the four-step procedure and
records the result; a REST endpoint mirrors it so the UI can clear a
marker after a manual fix.

WHY THE VERDICT CARRIES A CODE HASH. A verdict describes the code it was
checked against. Invalidating it on edit means deciding which edits count
— a when_to_use tweak shouldn't void a code check, a rewrite must — which
is fiddly and easy to get subtly wrong, and easy for a new write path to
forget entirely. Stamping the verdict with a hash sidesteps all of it: one
whose code_sha no longer matches is self-evidently expired, computed at
read time, no invalidation branch to maintain.

That makes "expired" the interesting filter case. It is not `drifted`
(nothing was found wrong) and not `unverified` (a check did happen), yet
it plainly needs looking at — so `verification=attention` covers both. To
keep that one index-served predicate rather than a post-filter that would
make the pagination total a lie, data now also mirrors the CURRENT code's
fingerprint as data.code_sha, and a jsonpath compares the two fields
within the row. The filter is implemented in both dialects, SQL and
Python, for the same reason the location filter is: the semantic arm's
candidates arrive already fetched.

A merge deliberately carries no verdict forward — the survivor's code is a
union of several sources, so no prior check describes it, and unverified
is the honest answer.

UI: a danger-toned drift badge on each card (an actively misleading record
outranks a merely unused one), and a "Needs attention" filter. Its empty
state says plainly that never-verified snippets don't appear there —
otherwise `attention` would mean "everything" on day one and be useless as
a worklist.

Refs #2086

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/views/SnippetListView.vue

@@ -34,6 +34,15 @@ function toggleLocationFilter() {
   if (!showLocationFilter.value && locationActive.value) clearLocation();
 }
 
+// Drift check (#2086) — "attention" is everything whose recorded location or
+// code no longer checks out, plus everything whose verdict expired because the
+// snippet was edited since it was checked.
+const needsAttentionOnly = ref(false);
+function toggleAttention() {
+  needsAttentionOnly.value = !needsAttentionOnly.value;
+  loadSnippets();
+}
+
 // Multi-select → merge
 const selectMode = ref(false);
 const selectedIds = ref<Set<number>>(new Set());
@@ -96,6 +105,7 @@ async function loadSnippets() {
       repo: locRepo.value.trim() || undefined,
       path: locPath.value.trim() || undefined,
       symbol: locSymbol.value.trim() || undefined,
+      verification: needsAttentionOnly.value ? "attention" : undefined,
     });
     snippets.value = data.snippets;
   } catch {
@@ -144,6 +154,39 @@ function usageBadge(s: SnippetListItem): string {
   return `${u.pull_count}/${u.surfaced_count} used`;
 }
 
+/** Short label for the drift verdict, or "" when there's nothing to say.
+ *  An expired verdict is reported as "unchecked" whatever it used to say —
+ *  it was about code that is no longer in the record. */
+function driftBadge(s: SnippetListItem): string {
+  const v = s.verification;
+  if (!v) return "";
+  if (!v.current) return "unchecked since edit";
+  return { ok: "", missing: "path gone", moved: "symbol moved", changed: "code drifted" }[
+    v.status
+  ] ?? "";
+}
+
+function driftTitle(s: SnippetListItem): string {
+  const v = s.verification;
+  if (!v) return "";
+  const when = v.checked_at
+    ? `Checked ${new Date(v.checked_at).toLocaleDateString()}`
+    : "Checked";
+  if (!v.current) {
+    return (
+      `${when}, but the snippet has been edited since — that verdict was about ` +
+      `code this record no longer holds. Re-verify it.`
+    );
+  }
+  const what =
+    {
+      missing: "the recorded path no longer exists",
+      moved: "the file is there but the symbol isn't in it",
+      changed: "the source no longer matches the recorded code",
+    }[v.status] ?? "";
+  return v.detail ? `${when}: ${what}. ${v.detail}` : `${when}: ${what}.`;
+}
+
 function usageTitle(s: SnippetListItem): string {
   const u = s.usage;
   if (!u) return "";
@@ -201,6 +244,15 @@ function usageTitle(s: SnippetListItem): string {
              a screen reader too. -->
         {{ locationActive ? "Location · filtering" : "Location" }}
       </button>
+      <button
+        class="btn-ghost"
+        :class="{ 'filter-on': needsAttentionOnly }"
+        :aria-pressed="needsAttentionOnly"
+        title="Show only snippets whose recorded location or code no longer checks out — including ones edited since they were last verified"
+        @click="toggleAttention"
+      >
+        {{ needsAttentionOnly ? "Needs attention · filtering" : "Needs attention" }}
+      </button>
     </div>
 
     <!-- Reverse lookup: what's already kept in this repo / file / symbol. -->
@@ -243,20 +295,27 @@ function usageTitle(s: SnippetListItem): string {
     <div v-else-if="snippets.length === 0" class="empty-state-rich">
       <div class="empty-icon">❭_</div>
       <p class="empty-title">
-        {{ locationActive
-          ? "Nothing kept at that location yet"
-          : search.trim()
-            ? "No snippets match your search"
-            : "No snippets kept yet" }}
+        {{ needsAttentionOnly
+          ? "Everything checks out"
+          : locationActive
+            ? "Nothing kept at that location yet"
+            : search.trim()
+              ? "No snippets match your search"
+              : "No snippets kept yet" }}
       </p>
       <p class="empty-sub">
-        {{ 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." }}
+        {{ needsAttentionOnly
+          ? "No snippet has drifted from its recorded location or code — as far as anything has been checked. Snippets nobody has verified yet don't appear here."
+          : 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">
+      <button v-if="needsAttentionOnly" class="empty-action" @click="toggleAttention">
+        Show all snippets
+      </button>
+      <button v-else-if="locationActive" class="empty-action" @click="clearLocation">
         Clear location filter
       </button>
       <button
@@ -295,6 +354,9 @@ function usageTitle(s: SnippetListItem): string {
         </p>
         <div class="card-footer">
           <span class="meta-date">Updated {{ new Date(s.updated_at).toLocaleDateString() }}</span>
+          <span v-if="driftBadge(s)" class="drift-tag" :title="driftTitle(s)">
+            {{ driftBadge(s) }}
+          </span>
           <span
             v-if="usageBadge(s)"
             class="usage-tag"
@@ -618,6 +680,17 @@ function usageTitle(s: SnippetListItem): string {
   color: var(--color-text-muted);
 }
 
+/* Drift is a stronger signal than dead weight: the record may be actively
+   misleading, not merely unused. Danger tone, and it sits first in the footer. */
+.drift-tag {
+  font-size: 0.7rem;
+  padding: 0.1rem 0.4rem;
+  border-radius: 4px;
+  white-space: nowrap;
+  background: color-mix(in srgb, var(--color-danger, #b91c1c) 15%, transparent);
+  color: var(--color-danger, #b91c1c);
+}
+
 .usage-tag {
   font-size: 0.7rem;
   padding: 0.1rem 0.4rem;