security: close a TOCTOU window when writing openfusion.yaml in setup

[shrdgn]

What & why

run_setup() (openfusion/cli.py) wrote the key-bearing openfusion.yaml
like this:

path.write_text(build_setup_yaml(preset, key), encoding="utf-8")
with contextlib.suppress(OSError):
    path.chmod(0o600)

Path.write_text creates the file via the process's default open() mode,
so under a permissive umask (e.g. 022) the file — which build_setup_yaml
embeds the plaintext OpenRouter API key into — is briefly world/group-readable
before the subsequent chmod(0o600) narrows it.

This is exactly the TOCTOU race openfusion/credentials.py's save_key()
was already hardened against: it opens with os.open(path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600) up front (with a comment explaining why),
and tests/test_credentials.py::test_key_file_created_private_even_if_chmod_fails
regression-tests it. run_setup() had the identical gap and no equivalent
test existed for it.

What changed

• run_setup() now opens openfusion.yaml with os.open(..., 0o600) before
  writing, mirroring credentials.save_key(). The chmod(0o600) afterward
  is kept as belt-and-suspenders for a pre-existing file from before this fix.
• Added two tests to tests/test_cli.py: one asserting the written config
  file is 0o600, and a regression test (mirroring test_credentials.py's
  chmod-failure case) confirming the file is created private even when
  Path.chmod raises.

How it was tested

• ruff check . passes
• pytest -q passes (no live network) — 480 passed
• New behavior has a test — test_run_setup_writes_config_file_private,
  test_run_setup_config_file_created_private_even_if_chmod_fails
• Docs updated if config / request surface / defaults changed — n/a, no
  surface change, only how an existing file gets written
• No secrets, prompts, or response bodies added to logs or metrics
• Quality/cost claims backed by a bench/run.py number — not applicable

Notes for reviewers

Only openfusion/cli.py's run_setup() changed (plus tests). Small,
self-contained, reuses an existing pattern already proven elsewhere in the
codebase.

---

Generated by Claude Code