Fix false unpushed commit warnings for worktree archive

[boudra]

Linked issue

None.

Type of change

• Bug fix
• New feature (with prior issue + design alignment)
• Refactor / code improvement
• Docs

What does this PR do

Fresh Paseo worktrees no longer show every commit in the repository history as "unpushed commits" when the user opens the archive warning.

The status check still preserves the original local-only branch behavior: when a branch has commits ahead of its base but no remote branch yet, those commits count as unpushed. The fix is that the fallback now compares against the known base ref instead of counting the entire branch history.

How did you verify it

Reproduced the bug with a red regression test for a fresh no-track Paseo worktree: before the fix it reported the full branch history as ahead even though it had no commits ahead of base.

Added coverage for both sides of the behavior:

• a fresh no-track worktree at its base reports aheadOfOrigin: 0
• a local-only worktree with one commit ahead of base reports aheadOfOrigin: 1
• the existing deleted-upstream case still reports aheadOfOrigin: null

Verified with:

• npx vitest run packages/server/src/utils/checkout-git.test.ts --bail=1
• npm run format:files -- packages/server/src/utils/checkout-git.ts packages/server/src/utils/checkout-git.test.ts
• npm run lint -- packages/server/src/utils/checkout-git.ts packages/server/src/utils/checkout-git.test.ts
• npm run typecheck
• npm run format

The commit hook also reran targeted lint, format check, and typecheck successfully.

Checklist

• One focused change. Unrelated cleanups split out.
• npm run typecheck passes
• npm run lint passes
• npm run format ran (Biome)
• UI changes include screenshots or video for every affected platform (N/A: no UI changes)
• Tests added or updated where it made sense