feat(skills): add commit message generator skill

.github/skills/commit-message-generator/SKILL.md

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+---
+name: commit-message-generator
+description: Generate appropriate commit messages based on Git diffs
+---
+
+## Prerequisites
+- This Skill retrieves Git diffs and suggests meaningful commit messages
+- Message format should follow Conventional Commits
+- Commit messages should be in English
+- **Never perform Git commit or Git push**
+
+## Steps
+1. Run `git status` to check modified files
+2. Retrieve diffs with `git diff` or `git diff --cached`
+3. Analyze the diff content and determine if changes should be split into multiple commits
+4. For each logical group of changes:
+   - List the target files
+   - Generate a message in English compliant with Conventional Commits
+   - Suggest the command: `git add <files> && git commit -m "<message>"`
+5. If changes are extensive and should be split, provide:
+   - Rationale for the split
+   - Multiple commit suggestions with their respective target files and messages
+
+## Commit Splitting Guidelines
+- Split commits when changes span multiple logical concerns (e.g., feature + refactoring)
+- Group related files that serve the same purpose
+- Keep each commit focused on a single, atomic change
+
+## Notes
+- **This Skill must never execute `git commit` or `git push`**
+- Only suggest commands; execution is entirely at user's discretion
+- Users must explicitly perform commits and pushes themselves