feat(hook): close mailbox on SessionEnd

New SessionEnd plugin hook runs `claude-mailbox session-end`, which derives the session's auto-name from stdin and asks the daemon to delete the mailbox if it has no pending messages either direction. Renamed mailboxes are preserved (the auto-name no longer exists, so DELETE is a no-op). The daemon-side `SKIP_UPSERT_PATHS` prevents the request from auto-recreating the mailbox. Sweeper remains the safety net for sessions that exit ungracefully.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

node/tests/db.test.ts

@@ -310,3 +310,54 @@ describe("pruneStale", () => {
     }
   });
 });
+
+describe("deleteIfEmpty", () => {
+  it("deletes a fresh mailbox with no pending messages and wipes its delivered history", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.send("alice", "bob", "old");
+      store.checkInbox("bob");
+      const r = store.deleteIfEmpty("bob");
+      expect(r).toEqual({ deleted: true, reason: "deleted" });
+      expect(store.listMailboxes().map((m) => m.name)).not.toContain("bob");
+    } finally {
+      store.close();
+    }
+  });
+
+  it("refuses to delete when the mailbox has undelivered incoming mail", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.send("alice", "bob", "still pending");
+      const r = store.deleteIfEmpty("bob");
+      expect(r).toEqual({ deleted: false, reason: "has-pending" });
+      expect(store.peek("bob").pending).toBe(1);
+    } finally {
+      store.close();
+    }
+  });
+
+  it("refuses to delete when the mailbox has undelivered outgoing mail", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.send("alice", "bob", "from alice");
+      const r = store.deleteIfEmpty("alice");
+      expect(r).toEqual({ deleted: false, reason: "has-pending" });
+      expect(store.listMailboxes().map((m) => m.name)).toContain("alice");
+    } finally {
+      store.close();
+    }
+  });
+
+  it("is a no-op for an unknown name (e.g. renamed mailbox)", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.upsertMailbox("alice");
+      const r = store.deleteIfEmpty("nope");
+      expect(r).toEqual({ deleted: false, reason: "not-found" });
+      expect(store.listMailboxes().map((m) => m.name)).toEqual(["alice"]);
+    } finally {
+      store.close();
+    }
+  });
+});