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/cmd/gossip.go

@@ -0,0 +1,50 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/david/knox/internal/db"
+	"github.com/david/knox/internal/watch"
+)
+
+// NewGossipCmd exposes peer status/diff for the gossip protocol.
+func NewGossipCmd(kdb *db.KnoxDB) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "gossip",
+		Short: "Inspect peer-to-peer observation sync",
+	}
+	cmd.AddCommand(
+		&cobra.Command{
+			Use:   "status",
+			Short: "Show known peers and knowledge vectors",
+			RunE: func(c *cobra.Command, args []string) error {
+				peers, err := kdb.ListPeers()
+				if err != nil {
+					return err
+				}
+				vector, err := kdb.KnowledgeVector()
+				if err != nil {
+					return err
+				}
+				fmt.Printf("node_id: %s\n", kdb.NodeID())
+				fmt.Printf("peers (static: %d configured):\n", len(watch.PeerAddrs()))
+				for _, p := range peers {
+					fmt.Printf("  %-24s %-28s last=%s  vector=%d\n", p.PeerID, p.Addr, p.LastHandshake, p.Cursor)
+				}
+				if len(peers) == 0 {
+					fmt.Println("  (none — set KNOX_PEERS to sync with other nodes)")
+				}
+				fmt.Println("knowledge vector (node_id -> max hcl):")
+				for nid, hcl := range vector {
+					if nid == kdb.NodeID() {
+						continue
+					}
+					fmt.Printf("  %-24s -> %d\n", nid, hcl)
+				}
+				return nil
+			},
+		},
+	)
+	return cmd
+}