✨ feat: add changelog-manager skill for better changelog maintenance

- Created changelog-manager skill to help agents properly maintain AGENT_CHANGELOG.md
- Provides guidance on when and how to update changelog
- Includes validation checks for format, content, and references
- Offers best practices for compact, outcome-focused entries
- Integrates with agent workflow for consistent documentation

This skill helps maintain the discipline of:
- Updating after each significant session
- Following consistent What/Why/How structure
- Linking to references (issues, ADRs, commits)
- Keeping entries compact and outcome-focused

Refs: #documentation, #changelog, #discipline

.vibe/skills/changelog-manager/references/REFERENCE.md

@@ -0,0 +1,50 @@
+# changelog-manager Reference
+
+## Overview
+
+Detailed technical reference for the changelog-manager skill.
+
+## Key Concepts
+
+### [Concept 1]
+
+[Detailed explanation]
+
+### [Concept 2]
+
+[Detailed explanation]
+
+## API Reference
+
+### [Function/Method Name]
+
+**Description**: [What it does]
+
+**Parameters**:
+-  - [Type]: [Description]
+-  - [Type]: [Description]
+
+**Returns**: [Return type and description]
+
+**Example**:
+```bash
+[example usage]
+```
+
+## Troubleshooting
+
+### [Issue 1]
+
+**Symptoms**: [What the user sees]
+
+**Cause**: [Root cause]
+
+**Solution**: [How to fix it]
+
+### [Issue 2]
+
+**Symptoms**: [What the user sees]
+
+**Cause**: [Root cause]
+
+**Solution**: [How to fix it]