Adds produce_bark() to src/infrastructure/presence.py that formats
chat responses as Matrix bark messages with:
- type='bark', agent_id, data={text, reply_to, style}, ts
- style defaults to 'speech', supports 'thought', 'whisper', 'shout'
- Text truncated to 280 characters
- Invalid styles fall back to 'speech'
Adds comprehensive tests for all bark functionality.
Fixes#671
