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>

frontend/src/components/graph/encoding.ts

@@ -28,6 +28,7 @@ export const ACTIVITY_TAG: Record<PodGraphActivityKind, { color: string; label:
   outcome: { color: GREEN, label: 'OUTCOME' },
   learned: { color: VIOLET, label: 'LEARNED' },
   agent: { color: BLUE, label: 'AGENT' },
+  suppressed: { color: VIOLET, label: 'SUPPRESSED' },
 };
 
 export const KIND_COLOR: Record<PodGraphNodeKind, string> = {