fix(sdk): support a published git status baseline

[matej21]

Summary

• Allow presets to configure the Git status baseline through gitStatusPlugin.configure({ baseBranch: "origin/main" }).
• Preserve existing default-branch detection when no baseline is configured.
• Return no snapshot when an explicit baseline is unavailable, rather than substituting a local branch or reporting zero.

A publish operation can advance local main before its remote push succeeds. Comparing status with that local branch then hides unpublished work. Hosts can now select their remote-tracking publication ref without replacing the plugin. The host remains responsible for fetching it.

Verification

• Real-Git regression tests cover a local main advanced ahead of origin/main, subsequent remote convergence, and a missing configured baseline through both the native process runner and GitClient adapter.
• Git status tests: 12 passed.
• bun run ts:build, SDK test typecheck, and bun run lint passed.