Fix/allow stale branch auto sync (#556)

* 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>

---------

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/sync_docs_analyze.yml

@@ -94,7 +94,13 @@ jobs:
 
           else
             echo "🆕 New PR event - analyzing full changes"
-            COMPARE_BASE="$PR_BASE"
+
+            # Use merge-base to find where branch diverged from main
+            # This allows stale branches to trigger automation without false "mixed content" errors
+            MERGE_BASE=$(git merge-base "$PR_BASE" "$PR_HEAD")
+            echo "Branch diverged from main at: $MERGE_BASE"
+
+            COMPARE_BASE="$MERGE_BASE"
             COMPARE_HEAD="$PR_HEAD"
             IS_INCREMENTAL="false"
           fi