feat(plugin): SessionStart hook discovers and announces active peers

session-announce now calls /v1/list with the session's X-Mailbox header,
which both registers the session with the daemon and returns all known
mailboxes in one round-trip. The output appends an "Active peers" block
listing mailboxes seen within the last hour (configurable via
--peer-window-minutes), capped at 10 entries by default. Self is
filtered out; the list is sorted most-recent-first.

So when the user says "I started a second session, coordinate with it",
Claude already has the peer's mailbox name in context — no manual
list_mailboxes call needed.

The peer-formatting logic is extracted into formatActivePeerList for
unit testing; CLI tests now pin --url to an unreachable port to keep
assertions stable on machines that have a real daemon running.

node/src/hook.ts

@@ -41,6 +41,41 @@ export function deriveSessionName(sessionId: string, base?: string | null): stri
   return `claude-${short}`;
 }
 
+export interface PeerEntry {
+  name: string;
+  lastSeenAt: string;
+}
+
+export function formatActivePeerList(
+  peers: PeerEntry[],
+  selfName: string,
+  options: { windowMinutes: number; maxPeers: number; now?: number },
+): string[] {
+  const others = peers.filter((p) => p.name !== selfName);
+  const cutoff = (options.now ?? Date.now()) - options.windowMinutes * 60_000;
+  const active = others
+    .filter((p) => {
+      const t = new Date(p.lastSeenAt).getTime();
+      return Number.isFinite(t) && t >= cutoff;
+    })
+    .sort((a, b) => b.lastSeenAt.localeCompare(a.lastSeenAt))
+    .slice(0, options.maxPeers);
+
+  if (active.length === 0) {
+    return [
+      `No other mailboxes seen within the last ${options.windowMinutes} minutes (${others.length} total registered).`,
+    ];
+  }
+
+  const lines = [
+    `Active peers (seen within last ${options.windowMinutes} min, ${active.length} of ${others.length} total):`,
+  ];
+  for (const p of active) {
+    lines.push(`  - ${p.name} (last seen ${p.lastSeenAt})`);
+  }
+  return lines;
+}
+
 export interface HookMessage {
   id: number;
   from: string;