feat: EXT-007 P2P message history gossip

After hello verification, the connecting peer sends history_request to
the first peer it meets (one per room, no fan-out). The responder queries
SQLite and replies with a history_chunk. Received history is stored via
INSERT OR IGNORE (mid dedup) and emitted as history_loaded IPC events.

- proto: MsgHistoryRequest/Chunk types, HistoryEntry, EvtHistoryLoaded,
  ComputeMsgID (sha256 content-addressed ID), MsgID field on ChatMessage
- store: ALTER TABLE ADD COLUMN msg_id + unique index migration (idempotent);
  RecentMessagesSince query (msg_id IS NOT NULL filter); msg_id persisted on save
- mesh: RequestHistoryFrom, HandleHistoryRequest, HandleHistoryChunk methods;
  historyRequested/historyFirstPeer state to ensure single-peer requests
- peer: dispatch history_request/history_chunk; RequestHistoryFrom after hello;
  stamp MsgID on incoming chat messages
- ipc: stamp MsgID on outgoing group chat messages
- EXTENSIONS.md: EXT-007 documented

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/ipc/ipc.go

@@ -214,6 +214,7 @@ func handleClient(conn net.Conn, mgr *netmgr.Manager) {
 			} else {
 				// Group chat → spec "chat" type: flat {type, mid, room, text, ts}
 				mid := randomHex(16)
+				msgID := proto.ComputeMsgID(n.Identity.PeerID(), cmd.Room, ts, cmd.Body)
 				wire, err := json.Marshal(proto.PeerMessage{
 					Type: proto.MsgChat,
 					Mid:  mid,
@@ -226,11 +227,12 @@ func handleClient(conn net.Conn, mgr *netmgr.Manager) {
 				}
 				n.Mesh.Broadcast(wire)
 				local := &proto.ChatMessage{
-					Mid:  mid,
-					From: n.Identity.PeerID(),
-					Room: cmd.Room,
-					Text: cmd.Body,
-					Ts:   ts,
+					Mid:   mid,
+					MsgID: msgID,
+					From:  n.Identity.PeerID(),
+					Room:  cmd.Room,
+					Text:  cmd.Body,
+					Ts:    ts,
 				}
 				n.Mesh.SaveMessage(local)
 				n.Mesh.Emit(proto.IpcMessage{