Adds `src/infrastructure/protocol.py` with a `MessageType` StrEnum
covering all 9 Matrix PROTOCOL.md message types (agent_state,
visitor_state, bark, thought, system_status, connection_ack, error,
task_update, memory_flash) and typed dataclasses with `to_json()` /
`from_json()` serialisation.
Includes comprehensive unit tests for round-trip serialisation and
`WSMessage.from_json()` dispatch to the correct subclass.
Fixes#667
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>