fix: run scheduled jobs in background tasks to prevent bus blocking

[sakhnenkoff]

Summary

Fixes #174.

AgentHandler.handle_scheduled currently awaits claude.run_command() directly, which blocks the event bus for the entire duration of a Claude execution (often 5-10+ minutes). This causes subsequent scheduled jobs to be delayed or missed entirely.

Changes:

• handle_scheduled now dispatches work via asyncio.create_task() and returns immediately
• New _run_scheduled method runs the actual Claude work under an asyncio.Semaphore(2) to cap concurrent executions
• New _task_done callback cleans up finished tasks and logs errors (prevents "Task exception was never retrieved" warnings)
• handle_webhook is unchanged — webhooks are fast and blocking is fine

Why not modify EventBus instead?

Changing the bus dispatcher to run events concurrently would break ordering guarantees, risk fire-and-forget exception leaks, and require verifying all handlers for concurrent safety. Scoping the fix to AgentHandler.handle_scheduled is minimal and safe.

Tests

4 new tests in tests/unit/test_events/test_concurrent_scheduled.py:

• test_handle_scheduled_returns_immediately — verifies dispatch completes within 1s even when Claude would take 10s
• test_scheduled_semaphore_limits_concurrency — verifies semaphore caps at 2
• test_scheduled_task_errors_are_logged_not_raised — confirms background errors are logged, not propagated
• test_webhook_handler_still_blocks — verifies webhook handling still awaits directly

All 538 existing tests continue to pass.