fix(auth): use atomic writes and error recovery for auth.json (#37)

[randomm]

Summary

• Atomic write pattern (temp file + rename) prevents corruption from interrupted writes
• Error recovery in all() returns empty dict instead of crashing on corrupt JSON
• Null check in codex.ts prevents TypeError on undefined auth

Fixes #37

Changes

• packages/opencode/src/auth/index.ts: atomic writes in set()/remove(), error recovery in all()
• packages/opencode/src/plugin/codex.ts: null safety check

[github-actions]

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