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/src/server.ts

@@ -2,7 +2,7 @@ import Fastify, { type FastifyInstance, type FastifyReply, type FastifyRequest }
 import { readFileSync } from "node:fs";
 import { join, dirname } from "node:path";
 import { fileURLToPath } from "node:url";
-import { MailboxStore, rowToMessage } from "./db.js";
+import { MailboxStore, RenameError, rowToMessage } from "./db.js";
 import type { DaemonConfig } from "./config.js";
 import { registerMcp } from "./mcp.js";
 
@@ -100,6 +100,25 @@ export async function buildServer(cfg: DaemonConfig, store: MailboxStore): Promi
     }));
   });
 
+  app.post<{ Body: { to?: string } }>("/v1/rename", async (req, reply) => {
+    const from = req.mailboxName!;
+    const to = (req.body?.to ?? "").trim();
+    if (!to) {
+      reply.code(400);
+      return { error: "to is required" };
+    }
+    try {
+      const r = store.rename(from, to);
+      return { from: r.from, to: r.to, messagesTransferred: r.messagesTransferred };
+    } catch (err) {
+      if (err instanceof RenameError) {
+        reply.code(err.reason === "target-exists" ? 409 : 400);
+        return { error: err.message, reason: err.reason };
+      }
+      throw err;
+    }
+  });
+
   await registerMcp(app, store);
 
   return app;