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

docs/gossip-protocol.md

@@ -249,6 +249,17 @@ reach a booting node. Config via env `KNOX_PEER_ADDR` / `KNOX_PEERS`.
 Verified e2e: two daemons, `diff` previewed 1655 peer-only fingerprints, `sync`
 pulled all and converged B to 1655 observations.
 
+**M5 — Prometheus metrics.** DONE. `/metrics` served on a dedicated port
+(`KNOX_METRICS_ADDR`, default `:8932`, separate from gossip). Uses
+`prometheus/client_golang`; ships with the Go runtime and process collectors.
+Gauge set: DB-derived gauges refreshed per scrape (`observations_total{source_id}`,
+`observations_last_24h`, `entries`, `projects`, `sessions`, `pending_reflections`,
+`threads_total{status}`, `peers`, `observations_by_node{node_id}`,
+`knowledge_max_hcl{node_id}`) plus live gossip counters
+(`pulls/pushes`, `observations_pulled/pushed`, `errors`) incremented during the
+anti-entropy sweep, and `knox_node_info{node_id,name}` for scrape identity
+(tracks issue #2).
+
 ## 9. Future Work (explicitly out of M1–M4)
 
 - mDNS / rendezvous peer discovery.