docs: Document direct file editing for Claude in model/workflow skills

[stack72]

• Update swamp-model and swamp-workflow skills to recommend direct file editing
• Claude can now use the Edit tool on YAML files instead of piping via stdin
• Workflow: get → Edit tool → validate

Closes #253

[github-actions]

Review: ✅ Approved

This is a documentation-only PR that simplifies the "Edit" sections in both swamp-model and swamp-workflow skill files. The changes recommend using Claude's native Edit tool for direct file editing rather than stdin/heredoc patterns.

What was reviewed:

• Code style compliance: N/A - documentation only, no TypeScript changes
• DDD principles: N/A - skill instruction files, not domain code
• Test coverage: N/A - documentation changes don't require tests
• Security: No concerns - no executable code changes

Observations:

1. The recommended workflow (get → Edit tool → validate) is clear and consistent across both files
2. Important warnings (e.g., "Never modify the id field") are preserved
3. Alternative methods (interactive editor, stdin) remain documented for users who need them
4. The swamp repo index tip for stale search results is included

The PR achieves its goal of streamlining Claude's workflow for editing model and workflow files while maintaining backward compatibility documentation.