Constrain worker logs to session output directories

[fazpu]

Summary

• replace spawn_agent output_path with filesystem-safe worker_label
• write worker stdout/stderr under the run session output directory
• reject removed/path-like output fields and document the new log contract

Verification

• uv run ruff format --check src
• uv run ruff check src
• uv run pyright src
• uv run pytest src/tests/test_agent_tools.py src/tests/test_worker_sessions.py src/tests/test_harness.py -q

Note

• Full uv run pytest src/tests -q currently has 2 unrelated console-rendering failures in test_stream_token_bold_rendering and test_stream_token_heading_rendering, both expecting Rich bold ANSI output.