[cleanup] Phase 6: Test coverage improvements

[AlexMikhalev]

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

[AlexMikhalev]

PR #635 created: adds 95 new tests across haystack_core (0->7), terraphim_onepassword_cli (0->26), terraphim_hooks (8->20), terraphim_sessions (19->42).