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/hook.test.ts

@@ -2,15 +2,18 @@ import { describe, it, expect } from "vitest";
 import { mkdtempSync, readFileSync, rmSync, writeFileSync, mkdirSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
+import { execFileSync } from "node:child_process";
 import {
   applyInstall,
   applyUninstall,
   buildHookCommand,
+  deriveProjectName,
   deriveSessionName,
   formatActivePeerList,
   formatMessagesForHook,
   parseHookStdin,
   readSettings,
+  sanitizeProjectName,
   shortSessionId,
   writeSettings,
   type PeerEntry,
@@ -205,7 +208,7 @@ describe("parseHookStdin", () => {
   });
 });
 
-describe("shortSessionId / deriveSessionName", () => {
+describe("shortSessionId", () => {
   it("takes first 8 hex chars from a UUID", () => {
     expect(shortSessionId("abc12345-de67-89f0-1234-567890abcdef")).toBe("abc12345");
   });
@@ -217,26 +220,73 @@ describe("shortSessionId / deriveSessionName", () => {
   it("falls back to a sanitized prefix for non-hex ids", () => {
     expect(shortSessionId("session-Test123")).toBe("sessiont");
   });
+});
 
-  it("derives anonymous name when no base", () => {
-    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef")).toBe("claude-abc12345");
+describe("sanitizeProjectName", () => {
+  it("lowercases and replaces non-alnum with dashes", () => {
+    expect(sanitizeProjectName("My Project!")).toBe("my-project");
   });
 
-  it("prepends base prefix when given", () => {
-    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef", "backend")).toBe(
-      "backend-abc12345",
-    );
+  it("collapses runs of separators", () => {
+    expect(sanitizeProjectName("foo  __ bar")).toBe("foo-bar");
   });
 
-  it("treats whitespace-only base as no base", () => {
-    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef", "  ")).toBe(
-      "claude-abc12345",
-    );
+  it("trims leading/trailing dashes", () => {
+    expect(sanitizeProjectName("--foo--")).toBe("foo");
   });
 
-  it("derives different names for different sessions with the same base", () => {
-    const a = deriveSessionName("aaaa1111-de67-89f0-1234-567890abcdef", "shared");
-    const b = deriveSessionName("bbbb2222-de67-89f0-1234-567890abcdef", "shared");
+  it("returns empty for purely non-alnum input", () => {
+    expect(sanitizeProjectName("---")).toBe("");
+    expect(sanitizeProjectName("")).toBe("");
+    expect(sanitizeProjectName(null)).toBe("");
+    expect(sanitizeProjectName(undefined)).toBe("");
+  });
+
+  it("caps long names", () => {
+    const out = sanitizeProjectName("a".repeat(120));
+    expect(out.length).toBeLessThanOrEqual(40);
+  });
+});
+
+describe("deriveProjectName", () => {
+  it("uses cwd basename when not in a git repo", () => {
+    // tmpdir is virtually never inside a git repo; basename is platform-dependent.
+    const got = deriveProjectName(tmpdir());
+    expect(got).toMatch(/^[a-z0-9-]+$/);
+  });
+
+  it("falls back to 'claude' when cwd is empty", () => {
+    expect(deriveProjectName("")).toBe("claude");
+    expect(deriveProjectName(null)).toBe("claude");
+    expect(deriveProjectName(undefined)).toBe("claude");
+  });
+
+  it("uses git toplevel basename when called from inside a repo", () => {
+    // The test harness itself runs inside the claude-mailbox checkout.
+    let inRepo = false;
+    try {
+      execFileSync("git", ["rev-parse", "--show-toplevel"], { encoding: "utf8", stdio: "pipe" });
+      inRepo = true;
+    } catch {
+      inRepo = false;
+    }
+    if (!inRepo) return; // CI without git in PATH — skip.
+    const got = deriveProjectName(process.cwd());
+    // Anywhere in the repo, we should resolve to the repo's basename — sanitized.
+    expect(got).toMatch(/^[a-z0-9-]+$/);
+    expect(got.length).toBeGreaterThan(0);
+  });
+});
+
+describe("deriveSessionName", () => {
+  it("composes <project>-<short>", () => {
+    const got = deriveSessionName("abc12345-de67-89f0-1234-567890abcdef", "");
+    expect(got).toBe("claude-abc12345");
+  });
+
+  it("derives different names for different sessions in the same project", () => {
+    const a = deriveSessionName("aaaa1111-de67-89f0-1234-567890abcdef", "");
+    const b = deriveSessionName("bbbb2222-de67-89f0-1234-567890abcdef", "");
     expect(a).not.toBe(b);
   });
 });