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/tests/hook.test.ts

@@ -7,11 +7,13 @@ import {
   applyUninstall,
   buildHookCommand,
   deriveSessionName,
+  formatActivePeerList,
   formatMessagesForHook,
   parseHookStdin,
   readSettings,
   shortSessionId,
   writeSettings,
+  type PeerEntry,
 } from "../src/hook.js";
 
 describe("formatMessagesForHook", () => {
@@ -239,6 +241,74 @@ describe("shortSessionId / deriveSessionName", () => {
   });
 });
 
+describe("formatActivePeerList", () => {
+  const NOW = new Date("2026-05-19T12:00:00.000Z").getTime();
+
+  const peer = (name: string, isoOffsetMinutes: number): PeerEntry => ({
+    name,
+    lastSeenAt: new Date(NOW - isoOffsetMinutes * 60_000).toISOString(),
+  });
+
+  it("excludes self from the list", () => {
+    const out = formatActivePeerList(
+      [peer("self", 1), peer("alice", 1)],
+      "self",
+      { windowMinutes: 60, maxPeers: 10, now: NOW },
+    );
+    const joined = out.join("\n");
+    expect(joined).not.toContain("self");
+    expect(joined).toContain("alice");
+  });
+
+  it("filters out peers older than the window", () => {
+    const out = formatActivePeerList(
+      [peer("recent", 5), peer("stale", 120)],
+      "self",
+      { windowMinutes: 60, maxPeers: 10, now: NOW },
+    );
+    const joined = out.join("\n");
+    expect(joined).toContain("recent");
+    expect(joined).not.toContain("stale");
+    expect(out[0]).toContain("1 of 2 total");
+  });
+
+  it("returns a no-peers message when nothing is active", () => {
+    const out = formatActivePeerList(
+      [peer("ancient", 9999)],
+      "self",
+      { windowMinutes: 60, maxPeers: 10, now: NOW },
+    );
+    expect(out).toHaveLength(1);
+    expect(out[0]).toMatch(/No other mailboxes seen within the last 60 minutes/);
+    expect(out[0]).toContain("1 total registered");
+  });
+
+  it("caps at maxPeers and sorts most-recent first", () => {
+    const out = formatActivePeerList(
+      [peer("p1", 30), peer("p2", 20), peer("p3", 10)],
+      "self",
+      { windowMinutes: 60, maxPeers: 2, now: NOW },
+    );
+    const joined = out.join("\n");
+    expect(joined).toContain("p3");
+    expect(joined).toContain("p2");
+    expect(joined).not.toContain("p1");
+    expect(out[0]).toContain("2 of 3 total");
+    expect(joined.indexOf("p3")).toBeLessThan(joined.indexOf("p2"));
+  });
+
+  it("ignores peers with invalid lastSeenAt", () => {
+    const out = formatActivePeerList(
+      [{ name: "garbage", lastSeenAt: "not-a-date" }, peer("ok", 5)],
+      "self",
+      { windowMinutes: 60, maxPeers: 10, now: NOW },
+    );
+    const joined = out.join("\n");
+    expect(joined).toContain("ok");
+    expect(joined).not.toContain("garbage");
+  });
+});
+
 describe("readSettings / writeSettings roundtrip", () => {
   it("survives an install → write → read cycle", () => {
     const dir = mkdtempSync(join(tmpdir(), "claude-mailbox-hook-"));