feat: M3 peer gossip protocol

Refs #1

- peers table (peer_id, addr, cursor, last_handshake)
- watch serves HTTP API: GET /v1/ping (knowledge vector),
  GET /v1/log?node=&after= (cursor-paged pull), POST /v1/obs/batch
- anti-entropy sweep (Run): ping, pull what we lack, push own obs;
  echo suppressed by node_id ownership; reconcile-on-pull
- config via KNOX_PEERS / KNOX_PEER_ADDR; knox gossip status
- db: GossipObservation wire type, PushObservations, ObservationsAfter,
  KnowledgeVector, peer upsert/list
- integration tests: bidirectional convergence + idempotency

docs/gossip-protocol.md

@@ -225,10 +225,14 @@ UNIQUE index) making auto-creation idempotent; `knox reconcile` rebuilds
 create/0 link on re-run); a log-only DB reconstructs `entries` and thread
 `cluster_key`s bit-identical to the original.
 
-**M3 — Peer protocol.** `peers` settings table, `/v1/ping`, `/v1/log` pull,
-`/v1/obs/batch` push, handshake + periodic anti-entropy, echo suppression.
-Verify: two nodes converge to identical logs after partition (integration test).
-`time.Now()`-free fact paths confirmed by grep.
+**M3 — Peer protocol.** DONE. `peers` table; HTTP API (`GET /v1/ping` with
+knowledge vector, `GET /v1/log?node=&after=` pull with cursor paging,
+`POST /v1/obs/batch` push); `watch` daemon serves its log and runs a periodic
+anti-entropy sweep (`KNOX_PEERS`, `KNOX_PEER_ADDR`), reconcile-on-pull; echo
+suppression by `node_id` ownership; `knox gossip status`. Verify: two nodes
+converge to identical logs after a bidirectional sweep (integration test
+`internal/watch/gossip_test.go`); second sweep is idempotent; `time.Now()`-free
+fact paths confirmed by grep.
 
 **M4 — Ops & UX.** `knox gossip` subcommand (status/diff), reconcile-on-pull,
 tombstoned thread handling in diff output, logging, config (env `KNOX_PEER_ADDR`,