Implements produce_system_status() in src/infrastructure/presence.py that
generates system_status messages for the Matrix frontend.
Features:
- agents_online: Count of agents with status != offline
- visitors: Count from WebSocket clients (_ws_clients)
- uptime_seconds: Time since APP_START_TIME
- thinking_active: Bool from thinking engine state and settings
- memory_count: Total memories from get_memory_stats()
All data collection uses graceful degradation (try/except blocks)
to ensure the function never crashes even when dependencies fail.
Includes comprehensive tests in tests/unit/test_presence.py covering:
- Message structure validation
- Data field types and non-negativity
- Graceful degradation on errors
- Timestamp format
Fixes#681
