Update Instructions Skill

Smart merge for CLAUDE.md and AGENTS.md instruction files.

What It Does

1. Reads existing instruction files (if present)
2. Parses SW-managed sections (marked with <!-- SW:SECTION:X -->)
3. Preserves user content (anything between or after SW sections)
4. Updates SW sections with latest template content
5. Writes merged result back to file

When to Use

• After running specweave refresh-marketplace
• After upgrading SpecWeave version (npm update -g specweave)
• When CLAUDE.md or AGENTS.md seem outdated
• To sync instruction files with latest framework features