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/watch/reconcile.go

@@ -0,0 +1,23 @@
+package watch
+
+import (
+	"fmt"
+
+	"github.com/david/knox/internal/db"
+)
+
+// Reconcile rebuilds all derived state from the observation log: entries are
+// reconstituted via deterministic SQL aggregation, then the auto-threader
+// re-links/re-creates threads idempotently by cluster_key. Returns the threads
+// created and observations linked.
+func Reconcile(kdb *db.KnoxDB) (int, int, error) {
+	if _, _, err := kdb.RebuildEntriesFromObservations(); err != nil {
+		return 0, 0, fmt.Errorf("rebuild entries: %w", err)
+	}
+	threader := NewAutoThreader(kdb)
+	created, linked, err := threader.AutoThread()
+	if err != nil {
+		return 0, 0, fmt.Errorf("auto-thread: %w", err)
+	}
+	return created, linked, nil
+}