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>

docs/cont_learning.md

@@ -98,6 +98,7 @@ Additive routes in `backend/src/server.ts` (shared file — additive only).
 | `collisions`      | **collision** nodes; `collides` (eng→col) + `touches` (file→col)           |
 | `interventions`   | **intervention** nodes; `warns` (col→intervention)                         |
 | `outcomes`        | `learned_from` (intervention→owner) on accepted; flips nodes to `learned`  |
+| `suppressions`    | `suppressed` activity beats — a dismissed signature recurred and PodMan stayed quiet (negative-feedback made visible; written at repeat time by the agent) |
 
 Metrics (learned owners / open risk paths / accept rate) are live counts.