refactor(agent): unify process_direct return contract

[bitifirefly]

Summary

• unify AgentLoop.process_direct() to return OutboundMessage | None instead of raw text
• update CLI call paths (gateway, agent, cron run, heartbeat) to consume outbound envelope content consistently
• update hardware runtime and digital task service to normalize direct-call outputs via safe text extraction
• align affected tests with the unified return contract

Validation

• ruff check on touched files
• targeted pytest for affected modules (66 passed)
• full suite: pytest -q (396 passed)