Add session rename support

[daniel-agentee]

Summary

• advertise sessionCapabilities.setTitle and handle session/setTitle via the generated Codex app-server thread/name/set request
• add a /rename built-in slash command that renames the current session and publishes it in available commands
• cover the capability, extension request, slash command behavior, and updated command snapshots

Notes

• session/setTitle is registered as an extension request until the ACP SDK exposes a typed method for it.

Validation

• npm run typecheck
• npm run build
• npm test

[daniel-agentee]

@codex review