[cleanup] Phase 6: Test coverage improvements #628

Summary

Add tests to zero-coverage crates.

Targets

• haystack_core -- core abstraction layer, needs unit tests
• terraphim_hooks -- hook execution, needs integration tests
• terraphim_sessions -- session management, needs unit tests
• terraphim_onepassword_cli -- 1Password integration (may need live service)

Approach

• #[tokio::test] unit tests inline with mod tests {}
• Integration tests in tests/ for cross-crate behavior
• Gate external-service tests behind env vars