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/watch.go

@@ -2,6 +2,7 @@ package watch
 
 import (
 	"log"
+	"net/http"
 	"os"
 	"path/filepath"
 	"strings"
@@ -12,7 +13,10 @@ import (
 	"github.com/fsnotify/fsnotify"
 )
 
-const browserInterval = 5 * time.Minute
+const (
+	browserInterval = 5 * time.Minute
+	gossipInterval  = 1 * time.Minute
+)
 
 type Watcher struct {
 	knoxDB        *db.KnoxDB
@@ -53,15 +57,31 @@ func (w *Watcher) Start() error {
 		log.Printf("[knox] obsidian vault: %s", w.vault)
 	}
 
+	// Gossip: serve our observation log to peers and periodically converge.
+	gossipAddr := ListenAddr()
+	if peers := PeerAddrs(); len(peers) > 0 {
+		node := &Node{Kdb: w.knoxDB, Name: "knox", Addr: gossipAddr}
+		srv := &http.Server{Addr: gossipAddr, Handler: node.Handler()}
+		go func() {
+			log.Printf("[knox] gossip listening on %s", gossipAddr)
+			if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+				log.Printf("[knox] gossip server: %v", err)
+			}
+		}()
+		log.Printf("[knox] gossip peers: %v", peers)
+	}
+
 	debounceMap := make(map[string]time.Time)
 	browserTicker := time.NewTicker(browserInterval)
 	giteaTicker := time.NewTicker(10 * time.Minute)
 	gitTicker := time.NewTicker(10 * time.Minute)
 	threadTicker := time.NewTicker(10 * time.Minute)
+	gossipTicker := time.NewTicker(gossipInterval)
 	browserRunning := false
 	giteaRunning := false
 	gitRunning := false
 	threadRunning := false
+	gossipRunning := false
 
 	for {
 		select {
@@ -119,6 +139,15 @@ func (w *Watcher) Start() error {
 				defer func() { threadRunning = false }()
 				w.autoThread()
 			}()
+		case <-gossipTicker.C:
+			if gossipRunning {
+				continue
+			}
+			gossipRunning = true
+			go func() {
+				defer func() { gossipRunning = false }()
+				w.syncGossip()
+			}()
 
 		case err, ok := <-watcher.Errors:
 			if !ok {
@@ -404,6 +433,33 @@ func (w *Watcher) autoThread() {
 	}
 }
 
+// syncGossip runs one anti-entropy sweep against configured peers and rebuilds
+// derived state if anything new arrived.
+func (w *Watcher) syncGossip() {
+	peers := PeerAddrs()
+	if len(peers) == 0 {
+		return
+	}
+
+	before := 0
+	if n, err := w.knoxDB.EntryCount(); err == nil {
+		before = n
+	}
+
+	Run(w.knoxDB, peers)
+
+	// If new observations arrived, reconcile to pick up entries/threads they
+	// imply (deterministic log → derived rebuild).
+	if after, err := w.knoxDB.EntryCount(); err == nil && after > before {
+		created, linked, err := Reconcile(w.knoxDB)
+		if err != nil {
+			log.Printf("[knox] gossip reconcile: %v", err)
+			return
+		}
+		log.Printf("[knox] gossip reconcile done: %d created, %d linked", created, linked)
+	}
+}
+
 func (w *Watcher) isRelevantEvent(event fsnotify.Event) bool {
 	if event.Has(fsnotify.Remove) || event.Has(fsnotify.Rename) {
 		return false