fix: close MCP client on session_shutdown so pi -p can exit

[d0ublecl1ck]

Problem

With this extension loaded, pi -p "ping" (non-interactive print mode) never exits after answering — the process hangs forever.

Root cause: the extension spawns the codebase-memory-mcp child process at load time (void register() → StdioClientTransport) but never closes it. The child's stdio pipes keep the parent Node event loop alive, so pi cannot exit after the answer completes.

Fix

Register an idempotent session_shutdown handler — the documented hook for session-scoped resources (docs/extensions.md: "Register an idempotent session_shutdown handler to close any session-scoped resources you start") — that closes the MCP client, letting the event loop drain.

pi.on("session_shutdown", async () => {
  await client?.close().catch(() => undefined);
  client = undefined;
});

Why this is safe

session_shutdown fires only at session end (process exit or session switch), not per conversation turn — interactive sessions keep the MCP connection alive across turns, so normal behavior is unchanged.

Verification

• Before: pi -p "ping" --provider <p> --model <m> hangs indefinitely after answering (verified against multiple providers).
• After: same command exits cleanly in seconds with the extension fully loaded and functional.