Implement a journal server that uses an Aedes MQTT broker to
persist stream messages to JSONL files and manage session metadata
via a manifest.
- Add `aedes`, `mqtt`, and `zod` dependencies
- Implement `createJournalServer` in `src/cli/journal.ts`
- Integrate journal server into the `serve` command
- Add `--mqtt-port` option to the CLI
- Add session lifecycle management (create, update, delete) via
retained MQTT topics
