fix(sdk): support an explicitly unpublished git baseline

[matej21]

Summary

• Extend Git status configuration with opt-in missingBase: "all" for a publication branch that does not exist yet.
• Count every HEAD commit only after proving the baseline ref is absent. Existing-but-unreadable refs still yield no snapshot.
• Pass the same explicit policy through GitCountAheadOptions and extend adapter conformance coverage.

This completes the initial-publication case for the configurable baseline added in #32. Default behavior is unchanged.

Verification

• Real-Git tests cover an absent remote baseline with local main already advanced, the remote ref appearing later, and a damaged existing baseline. Both native Git and the GitClient path are covered.
• Git status and adapter-conformance tests: 28 passed.
• bun run ts:build, SDK test typecheck, and bun run lint passed.