- Replace AgeKeySource+retrieve() with IdentitySource+load() returning Box<dyn Identity>
- Rewrite bitwarden/file/prompt sources to implement new trait
- Add resolve_identities() public API with partial-success semantics
- Delete _legacy.rs and remove legacy re-exports from lib.rs
- Update integration tests to use new API
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>