Review credential and cache handling before wider release

[rain2day]

Goal

Do a focused privacy/security review before promoting the project beyond early OSS status.

Review Areas

• Codex auth read and refresh write-back.
• Claude file, environment, and Keychain credential paths.
• Local cache contents under Application Support.
• Diagnostic logging to /tmp/codex-usage-halo.log.
• Provider backoff and refresh behavior.

Done When

• docs/PRIVACY.md reflects the verified behavior.
• SECURITY.md reporting guidance is still accurate.
• No logs or cache paths include raw tokens or full credential payloads.