feat(graph): record + surface suppressed-repeat activity at repeat time (Feature A) (#43)

Codex review fix. The earlier approach synthesized a 'suppressed' beat from every
dismissed OUTCOME, which is wrong: a dismissal is not a later suppressed repeat,
and the old dismissal timestamp sank below the 12-row activity cap (invisible).

Now the negative-feedback loop is recorded WHEN IT HAPPENS. When shouldIntervene()
returns false specifically because a signature was dismissed before
(agent/podman.ts), the agent writes a durable SuppressionDoc to a new
`suppressions` collection, timestamped at the repeat. graph/live.ts materializes
those into kind:'suppressed' activity (recent -> surfaces at the top). The
suppressed collision is never written to `collisions` (agent returns before
recordCollision), so this is its own record.

Verified end-to-end: a transient demo-pod suppression renders at the TOP of the
activity stream; cleaned up after. shared build + backend/frontend typecheck +
eslint pass.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/graph/live.ts

@@ -457,6 +457,31 @@ export async function materializePodGraph(podId: string): Promise<PodGraph | nul
     );
   }
 
+  // 7. Suppressed repeats: PodMan stayed quiet on a recurring collision because
+  // the signature was dismissed before — the negative-feedback loop made visible
+  // (Feature A). Stamped at repeat time, so it sorts as recent activity.
+  const suppressionDocs = await c.suppressions
+    .find({ podId })
+    .sort({ suppressedAt: -1 })
+    .limit(20)
+    .toArray();
+  for (const s of suppressionDocs) {
+    const sFile = normalizeFile(s.file);
+    if (!isFilePath(sFile)) continue;
+    const sEngs = (s.engineers ?? []).join(' + ') || 'teammates';
+    pushActivity(
+      activity,
+      {
+        id: `suppressed:${s.id}`,
+        at: s.suppressedAt,
+        kind: 'suppressed',
+        title: `Suppressed — ${shortLabel(sFile)} repeat silenced`,
+        detail: `${sEngs} on ${sFile} recurred, but it was dismissed before — PodMan stayed quiet.`,
+      },
+      activityIds,
+    );
+  }
+
   // Prune test-artifact engineers, then anything left orphaned by that.
   const dropNode = (id: string) => {
     b.nodes.delete(id);