✨ 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/scripts/example.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+# Example script for changelog-manager skill
+
+set -e
+
+echo "This is an example script for the changelog-manager skill"
+echo "Replace this with your actual script logic"
+
+# Your script implementation goes here
+# Example:
+# echo "Processing..."
+# [command] [arguments]