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

main.go

@@ -64,6 +64,7 @@ and maintains a searchable index. Use 'knox watch' for daemon mode.`,
 	root.AddCommand(knoxcmd.NewGiteaCmd(kdb))
 	root.AddCommand(knoxcmd.NewGitCmd(kdb))
 	root.AddCommand(knoxcmd.NewReconcileCmd(kdb))
+	root.AddCommand(knoxcmd.NewGossipCmd(kdb))
 	root.AddCommand(newServeCmd(kdb))
 
 	// MCP server subcommand