Treat agent output_path as a log stem

[fazpu]

Summary

• treat spawn_agent(output_path=...) as a semantic log stem instead of an exact stdout file path
• write worker stdout/stderr beside the stem as <stem>.stdout.jsonl and <stem>.stderr.log
• pass explicit stdout/stderr paths into the subprocess spawner
• add regression coverage for the artifact-directory collision that can occur when a worker writes <stem>/review.md

Verification

• uv run pytest src/tests/test_agent_tools.py::test_spawn_agent_schema_exposes_resume_fields src/tests/test_agent_tools.py::test_spawn_agent_treats_output_path_as_log_stem
• uv run pytest src/tests/test_agent_tools.py src/tests/test_worker_sessions.py src/tests/test_harness.py
• uv run ruff format --check . && uv run ruff check .
• uv run --extra dev pyright

Notes

• Intentionally does not add a defensive _tail_text() fallback; if a log path is still wrong, manifest generation should fail loudly.
• Full uv run pytest currently has two unrelated console rendering failures in src/tests/test_console.py around Rich bold ANSI output; the changed areas pass.