Harden worker file-write routing

[pmbstyle]

Summary

• harden worker routing so file-writing tasks prefer templates with filesystem write tools
• require workers to actually call fs_write before claiming workspace file completion
• fix Docker shared-workspace path handling for allowed worker writes
• add timeout enforcement around provider calls and small Windows/A2A robustness fixes

Root cause

Live A2A QA found that a worker could be selected without write tools, or could report a file as written while only writing inside temporary worker scratch. In Docker mode that scratch was cleaned up, so the parent agent saw a successful worker but the expected workspace artifact was missing.

Validation

• uv run pytest tests\test_worker_router.py tests\test_worker_launcher_isolation.py tests\test_filesystem_hardening.py tests\test_litellm_provider_payload_hardening.py tests\test_agent_worker_contracts.py tests\test_heartbeat.py
• full A2A core suite passed on the test instance: 6/6