Add CPAN report publishing skill

[fglock]

Summary

• add a publish-cpan-reports skill for lock-consistent snapshots of the four generated compatibility report files
• document the narrow dirty-tree exception for generated CPAN report publication
• allow documentation-only changes to skip the full build while requiring Markdown link validation
• record the shell-backtick incident in the project guidance document rather than in the skill

Validation

• skill validator: pass
• make check-links: pass, 0 errors
• direct offline link check for AGENTS.md and the new skill: pass, 0 errors
• full build and runtime tests not run; documentation and non-executable skill instructions only