* fix redirect language code prefixes
* fix: Allow stale branches to trigger auto-sync without false mixed-content errors
Changes the PR analysis to use merge-base instead of PR base commit for
comparison. This allows stale branches to trigger translation automation
without being falsely flagged as "mixed content" PRs.
**Problem:**
- When a PR branch becomes stale (main moves forward after branch creation)
- Old approach compared PR_BASE (main's current HEAD) vs PR_HEAD
- This included unrelated changes from main as "part of the PR"
- Caused false "mixed content" errors (e.g., PR #555)
**Solution:**
- Use merge-base to find where branch diverged from main
- Compare MERGE_BASE vs PR_HEAD (only actual PR changes)
- Ignore unrelated changes that landed on main after branch creation
**Benefits:**
- ✅ Better contributor experience (no manual branch updates needed)
- ✅ Accurate PR categorization (no false positives)
- ✅ Faster automation (no waiting for updates)
- ✅ Works with concurrent PRs
**Example (PR #555):**
- OLD: Detected 4 files (en + cn + jp) → "mixed content" ❌
- NEW: Detected 1 file (en only) → "english" ✅🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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Co-authored-by: Claude <noreply@anthropic.com>
