feat: Prometheus metrics endpoint for knox nodes

Refs #2

- /metrics served on a dedicated port (KNOX_METRICS_ADDR, default
  localhost:8932) via prometheus/client_golang, with Go runtime +
  process collectors
- DB-derived gauges refreshed per scrape: observations by source,
  last-24h observations, entries, projects, sessions, pending
  reflections, threads by status, peers, observations by origin node,
  knowledge vector (max hcl per node)
- live gossip counters (pulls/pushes, observations pulled/pushed,
  errors) incremented during the anti-entropy sweep; Run accepts an
  optional metrics handle (nil for one-shot CLI)
- knox_node_info{node_id,name} for scrape identification
- internal/metrics package + db MetricsSnapshot; tests for snapshot,
  scrape output, and counter increments

internal/cmd/gossip.go

@@ -61,7 +61,7 @@ func NewGossipCmd(kdb *db.KnoxDB) *cobra.Command {
 				if len(peers) == 0 {
 					return fmt.Errorf("no peers configured (set KNOX_PEERS)")
 				}
-				watch.Run(kdb, peers)
+				watch.Run(kdb, nil, peers)
 				created, linked, err := watch.Reconcile(kdb)
 				if err != nil {
 					return err