📝 docs: update commit-message skill with multi-issue closing best practices

Added critical documentation about using separate 'Closes' lines for PR merge commits:
- Explains why single-line multiple issue references fail
- Provides correct multi-line format examples
- Prevents common issue where only first issue gets closed
- Updates usage examples with proper multi-issue syntax

This fixes the issue we encountered where 'Closes #4, #5, #6' only closed #4.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

.vibe/skills/commit-message/SKILL.md

@@ -52,7 +52,7 @@ git commit -m "✨ feat: implement BDD testing framework"
 
 ### Issue References
 ```bash
-# When closing an issue
+# When closing a single issue
 git commit -m "✨ feat: implement workflow optimization (closes #2)"
 
 # When fixing a bug
@@ -63,6 +63,14 @@ git commit -m "📝 docs: update workflow documentation (related to #2)"
 
 # When referencing for context
 git commit -m "♻️ refactor: clean up CI code (see #3)"
+
+# For PR merges closing multiple issues (USE SEPARATE LINES!)
+git commit -m "✨ merge: implement authentication system
+
+Closes #4
+Closes #5
+Closes #6
+Refs #7, #8"
 ```
 
 ### Bug Fix
@@ -139,6 +147,29 @@ Example: ✨ feat: implement workflow (closes #2)
 **GitHub/Gitea Compatible:**
 These formats are recognized by both GitHub and Gitea to automatically close issues.
 
+### ⚠️ IMPORTANT: Multiple Issue Closing
+
+**For PR merge commits that close multiple issues, use SEPARATE lines:**
+
+```markdown
+✨ merge: implement authentication system
+
+Closes #4
+Closes #5  ← Use separate lines!
+Closes #6  ← This ensures ALL issues are closed
+Refs #7, #8
+```
+
+**❌ Avoid this (only closes first issue):**
+```markdown
+✨ merge: implement authentication system
+
+Closes #4, #5, #6  ← Only #4 gets closed!
+Refs #7, #8
+```
+
+**Why this matters:** GitHub/Gitea issue trackers typically only process the FIRST issue reference when multiple issues are listed on the same line. Using separate lines ensures ALL referenced issues are properly closed.
+
 ## Git Hooks for Code Quality
 
 The project includes Git hooks that automatically run before commits to ensure code quality: