Module credential 

Multi-source credential resolution.

Reads API keys from multiple sources with clear priority:

1. config.toml → [engine].api_key (explicit override)
2. ~/.oxi/auth.json (shared with oxi CLI if installed)
3. oxi-ai env var fallback (CI/CD, containers)

Handles legacy oxi-cli auth.json entries ({"type":"api_key","key":"..."}) by auto-migrating them to the TokenBundle format on first write.

Structs

CredentialStore

Multi-source credential resolver.

Enums

CredentialSource

Where a credential was found.