docs(schema): update docs for #54 — proto registry + buf CI gate

- packages/shared-types/README.md: new — documents HTTP vs event surfaces,
  proto file layout, schema evolution rules, and how to run buf locally
- ml/serving/README.md: note pydantic payload validation in consumer section
- CLAUDE.md: replace "schema registry enforced when #54 lands" with
  the actual state; remove #54 from active-work list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ml/serving/README.md

@@ -41,6 +41,8 @@ On startup, `nats_consumer.py` registers two durable push consumers against NATS
 - `signals.task.synced` — writes `{last_sync_ts, task_count}` to `{STATE_DIR}/{user}_sync.json`
 - `signals.tip.feedback` — logged for observability; reward update happens via the HTTP path in the recommender
 
+**Payload validation:** each message is validated against the pydantic models in `schemas.py` (mirroring `packages/shared-types/events/oo/events/v1/`). A `ValidationError` triggers a nak so the message is redelivered rather than silently dropped.
+
 **Ack semantics:** explicit ack on success; nak for redelivery on error; dead-lettered after `NATS_MAX_DELIVER` attempts.
 
 **Disabled** when `NATS_URL` is unset (default in local dev without NATS). No import of `nats-py` occurs in that case.