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

internal/db/schema.go

@@ -113,6 +113,16 @@ CREATE TABLE IF NOT EXISTS settings (
     value TEXT
 );
 
+-- PEERS: other knox nodes known to this one (gossip ring)
+CREATE TABLE IF NOT EXISTS peers (
+    peer_id        TEXT PRIMARY KEY,       -- remote node_id
+    addr           TEXT,                   -- e.g. http://192.168.1.20:8931
+    name           TEXT,
+    last_handshake TEXT,
+    cursor         INTEGER DEFAULT 0,      -- remote's max_hcl we've seen (knowledge)
+    created_at     TEXT DEFAULT (datetime('now'))
+);
+
 CREATE INDEX IF NOT EXISTS idx_threads_status ON threads(status);
 
 CREATE TABLE IF NOT EXISTS thread_observations (