feat: swarm membership discovery via peer-list gossip

Refs #1

- /v1/ping now advertises the node's known peers (peer_id, addr, name)
- Run sweeps static KNOX_PEERS + persisted discovered peers, enqueueing
  newly-learned addresses for direct sweeps (membership-only relay; no
  observation relay)
- db: ShareablePeers, SwarmPeerAddrs, MergePeer (cursor-preserving
  discovery upsert), MaxHCLForNode
- integration test: a node configured with a single seed discovers and
  pulls from other swarm members without direct configuration

docs/gossip-protocol.md

@@ -125,14 +125,16 @@ a single node the rowid remains monotonic, so the current dedup query
 
 ### 5.1 Transport
 
-Plain HTTP/JSON on a per-node advertized address (default port `8931`). Nodes
-discover peers via a static list in `settings` (M3). mDNS/rendezvous is future
-work.
+Plain HTTP/JSON on a per-node advertized address (default port `8931`). Peers
+are seeded from a static list (`KNOX_PEERS`), then the swarm discovers itself:
+each node advertises its known peer addresses in `/v1/ping`, and every sweep
+enqueues newly-learned nodes for direct contact (membership gossip — no relay of
+observations). mDNS/rendezvous is future work.
 
 Endpoints:
 
 ```
-GET  /v1/ping                       → { node_id, name, max_hcl }
+GET  /v1/ping                       → { node_id, name, max_hcl, peers: [{peer_id, addr, name}] }
 GET  /v1/log?after=<seq>&node=<id>  → { cursor, rows: [observation...] }   # pull
 POST /v1/obs/batch                  → body: [observation...]; reply: { accepted n, conflict n }  # push
 GET  /v1/diff                       → divergence summary (M4)
@@ -142,6 +144,11 @@ GET  /v1/diff                       → divergence summary (M4)
 
 - **Knowledge vector:** each node tracks `peer_id → max_hcl consumed`. Anti-entropy
   is a pull: periodically (and on handshake) query each peer's `/v1/log?after=...`.
+- **Membership gossip:** `/v1/ping` includes the responding node's known peers
+  (`peer_id`, `addr`, `name`). The caller merges them into its `peers` table and
+  enqueues their addresses for direct sweeps. A new node therefore joins the
+  whole swarm by configuring just one seed peer. Membership flows independently
+  of data — a node never relays another's observations, only its address.
 - **Push:** on a new local observation, best-effort `POST /v1/obs/batch` to known
   peers. A node does **not** re-broadcast something it merely received (that peer
   already has it and will pull from its origin) — this is the echo/loop