fix(auth): write Anthropic OAuth token files atomically to prevent corruption

[maymuneth]

What does this PR do?

agent/anthropic_adapter.py writes Anthropic OAuth token files using
write_text() directly — a non-atomic operation. If the process is
killed mid-write (crash, OOM, SIGKILL), the file is left partially
written and corrupt.

On the next startup, json.loads() fails to parse the corrupt token
file — the user's Anthropic OAuth session is permanently broken and
they must re-authenticate manually.

Two write sites are affected:

• _HERMES_OAUTH_FILE — the main Hermes OAuth token store
• cred_path — per-credential token files

Fix

Write to a .tmp sibling file first, then use Path.replace() which
is atomic on POSIX systems (rename syscall).

# Before (non-atomic)
_HERMES_OAUTH_FILE.write_text(json.dumps(data, indent=2), encoding="utf-8")

# After (atomic)
_tmp = _HERMES_OAUTH_FILE.with_suffix(".tmp")
_tmp.write_text(json.dumps(data, indent=2), encoding="utf-8")
_tmp.replace(_HERMES_OAUTH_FILE)

This is consistent with the atomic write pattern already used in
hermes_cli/auth.py (_save_auth_store), gateway/run.py, and
tools/skill_manager_tool.py.

Type of Change

• 🐛 Bug fix (reliability)

Checklist

• Read the Contributing Guide
• Commit messages follow Conventional Commits
• Consistent with existing atomic write pattern in Hermes
• No behavior change under normal operation
• Prevents OAuth token loss after process crash

[teknium1]

Thanks for this fix, @maymuneth — the atomic write pattern you introduced has been merged into main via the consolidated OAuth salvage PR #15175 (merged 2026-04-24).

Evidence:

• agent/anthropic_adapter.py lines 729-731 on main now use cred_path.with_suffix('.tmp') + .replace() — exactly the fix from this PR (commit 56086e3fd).
• The second hunk targeting _save_hermes_oauth_credentials / _HERMES_OAUTH_FILE was intentionally dropped during salvage: that function was already removed from anthropic_adapter.py by PR chore: remove unused Hermes-native PKCE OAuth flow #3107, so there was no non-atomic write left to fix at that site.
• PR fix(anthropic): consolidated OAuth — atomic writes, cc- prefix, stale detection + Keychain, aux 401 refresh #15175 body explicitly credits this PR (fix(auth): write Anthropic OAuth token files atomically to prevent corruption #5095) and documents both decisions.

This is an automated hermes-sweeper review. Closing as implemented on main.