fix: --scope changed should detect commits relative to remote base

[skoshx]

Summary

Fixes #1674 by improving the auto-detection of the base branch for --scope changed when no explicit --base flag is provided.

Root Cause

When running npx react-doctor --scope changed without an explicit --base flag, the defaultBranch() function would return the local branch name (e.g., "main"). When comparing currentBranch === baseBranch by name, the code would fall into the "uncommitted changes only" path, missing any committed changes.

This affected two scenarios:

1. Feature branches without a local main: Users on a feature branch with no local main branch (only origin/main) would get null from diffSelection because git merge-base main HEAD failed
2. Local main ahead of origin: Users on local main with committed changes ahead of origin/main would only see uncommitted working-tree changes

Fix

The defaultBranch() function now:

1. Checks if the remote tracking branch (origin/<branch>) exists
2. Returns origin/<branch> instead of <branch> when it does
3. Falls back to the old behavior when no remote exists

This ensures:

• --scope changed on a feature branch auto-compares against origin/main
• --scope changed on local main ahead of origin/main auto-compares against the remote
• The name comparison currentBranch === baseBranch correctly fails, triggering the merge-base path

Testing

Added two regression tests:

1. Feature branch with no local main - detects committed changes
2. Local main ahead of origin/main - detects committed changes

All core tests pass (2368/2368).

Scope

This change only affects local development when using --scope changed WITHOUT --base. It does NOT affect:

• CI workflows (GitHub Action provides explicit base via REACT_DOCTOR_BASE_SHA or --changed-files-from)
• Explicit --base usage (already worked correctly)
• Parity testing (uses explicit refs)

Closes #1674

  

[pkg-pr-new]

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commit: 6e628f6

[github-actions]

Interactive terminal E2E

Terminal Control verified the built CLI at 6e628f6 in a real PTY:

• selected a project interactively and observed Scanning... before the three-second Git delay completed
• waited for the clean result and exercised the compact report
• opened copy context and the GitHub Actions confirmation, then cancelled safely

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