Redesign worker child suspension

[pmbstyle]

Summary

• move child-worker join semantics out of the agent prompt loop and into WorkerRuntime suspend/resume handling
• add explicit worker/runtime protocol messages for awaiting and resuming child batches
• treat waiting-for-children as a first-class live worker state and cover the new race with targeted tests

Testing

• uv run pytest tests/test_worker_sdk.py tests/test_worker_status_tools.py tests/test_worker_suspend_resume.py tests/test_agent_loop_improvements.py tests/test_runtime_mcp_integration.py
• uv run pytest tests/test_parallel_worker_orchestration.py tests/test_worker_child_spawning.py tests/test_worker_yield_tool.py
• uv run ruff check src/octopal/runtime/workers/agent_worker.py src/octopal/runtime/workers/contracts.py src/octopal/runtime/workers/runtime.py src/octopal/tools/workers/management.py src/octopal/worker_sdk/protocol.py src/octopal/worker_sdk/worker.py tests/test_agent_loop_improvements.py tests/test_worker_sdk.py tests/test_worker_status_tools.py tests/test_worker_suspend_resume.py