Agents misdiagnosed test failures (#1652) because they didn't understand that files_manager defers all writes until context exit — subprocesses can't see uncommitted in-memory changes.

• files_manager() docstring (_config_file.py): Explains locking on entry, lazy reads, atomic flush on exit, and the subprocess visibility constraint.
• FileManager class docstring (_file/manager.py): Adds a numbered lifecycle (Enter → Read → Commit → Exit) with explicit subprocess guidance.
• usethis-python-test skill (SKILL.md v1.1): New "Using files_manager in tests" section with correct/incorrect patterns and a decision rule.

Key pattern the skill now teaches:

# Correct: flush to disk before subprocess reads config
with change_cwd(tmp_path), files_manager():
    use_ruff()

call_uv_subprocess(["run", "ruff", "check", "."], change_toml=False)

# Wrong: subprocess runs inside context, config not yet on disk
with change_cwd(tmp_path), files_manager():
    use_ruff()
    call_uv_subprocess(["run", "ruff", "check", "."], change_toml=False)