feat(naming)!: auto-derive mailbox name from project + runtime rename

Mailbox names are now built as <project>-<session-short>, where <project>
is the sanitized git-repo basename (or cwd basename) — no more env-var
prefix step. Sessions can re-tag themselves at runtime via the new
mcp__mailbox__rename tool (POST /v1/rename), which transfers all
pending messages to the new name in a single transaction. Peers using
the old name re-discover via list_mailboxes.

BREAKING: \$CLAUDE_MAILBOX_NAME is no longer read. Existing setups that
relied on the env-var prefix should remove it from .claude/settings.json;
the prefix now comes from the working directory automatically.

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

node/tests/db.test.ts

@@ -2,7 +2,7 @@ import { describe, it, expect, afterEach, beforeEach } from "vitest";
 import { mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { MailboxStore } from "../src/db.js";
+import { MailboxStore, RenameError } from "../src/db.js";
 
 let dir: string;
 let dbPath: string;
@@ -75,6 +75,93 @@ describe("send / peek / check round-trip", () => {
   });
 });
 
+describe("rename", () => {
+  it("renames a mailbox and transfers undelivered messages", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.send("alice", "bob-old", "hi");
+      store.send("alice", "bob-old", "again");
+
+      const r = store.rename("bob-old", "bob-new");
+      expect(r.from).toBe("bob-old");
+      expect(r.to).toBe("bob-new");
+      expect(r.messagesTransferred).toBe(2);
+
+      // Old name is gone.
+      const list = store.listMailboxes().map((m) => m.name);
+      expect(list).toContain("bob-new");
+      expect(list).not.toContain("bob-old");
+
+      // Messages still pending under the new name.
+      const peek = store.peek("bob-new");
+      expect(peek.pending).toBe(2);
+
+      // checkInbox under the new name yields the original bodies and the original from.
+      const pulled = store.checkInbox("bob-new");
+      expect(pulled.map((m) => m.body)).toEqual(["hi", "again"]);
+    } finally {
+      store.close();
+    }
+  });
+
+  it("also rewrites the from-side when the renamed mailbox was a sender", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.send("sender-old", "bob", "msg-1");
+      store.rename("sender-old", "sender-new");
+      const pulled = store.checkInbox("bob");
+      expect(pulled).toHaveLength(1);
+      expect(pulled[0]!.from_mailbox).toBe("sender-new");
+    } finally {
+      store.close();
+    }
+  });
+
+  it("treats rename-to-same-name as a no-op touch", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.upsertMailbox("alice");
+      const r = store.rename("alice", "alice");
+      expect(r.messagesTransferred).toBe(0);
+      expect(store.listMailboxes().map((m) => m.name)).toEqual(["alice"]);
+    } finally {
+      store.close();
+    }
+  });
+
+  it("rejects when target already exists", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      store.upsertMailbox("alice");
+      store.upsertMailbox("bob");
+      expect(() => store.rename("alice", "bob")).toThrow(RenameError);
+      try {
+        store.rename("alice", "bob");
+      } catch (e) {
+        expect((e as RenameError).reason).toBe("target-exists");
+      }
+      // Source still present after the failed attempt.
+      expect(store.listMailboxes().map((m) => m.name)).toEqual(["alice", "bob"]);
+    } finally {
+      store.close();
+    }
+  });
+
+  it("rejects when source is missing", () => {
+    const store = new MailboxStore(dbPath);
+    try {
+      try {
+        store.rename("nope", "fresh");
+      } catch (e) {
+        expect(e).toBeInstanceOf(RenameError);
+        expect((e as RenameError).reason).toBe("source-missing");
+      }
+    } finally {
+      store.close();
+    }
+  });
+});
+
 describe("listMailboxes", () => {
   it("returns mailboxes alphabetically with pendingForYou for the caller", () => {
     const store = new MailboxStore(dbPath);