[skills] Add packaged AGENTS.md skill (#128)

[coisa]

Related Issue

Closes #128

Motivation / Context

• Package a reusable Fast Forward skill for repository-level AGENTS.md authoring.
• Make the workflow available to synced consumer repositories without adding new synchronization code.

Changes

• Add a packaged create-agentsmd skill with focused guidance and a reference checklist.
• Add Codex UI metadata for the new packaged skill.
• Update repository guidance and changelog entries to mention the new packaged skill.

Verification

• composer dev-tools
• Focused command(s): composer dev-tools tests -- --filter=SkillsSynchronizerTest
• Manual verification: confirmed the new skill lives under .agents/skills/ and relies on the existing directory-based synchronization flow.

Documentation / Generated Output

• README updated
• docs/ updated
• Generated or synchronized output reviewed

Changelog

• Added a notable CHANGELOG.md entry

Reviewer Notes

• The skill-creator quick validator could not be used directly in this environment because PyYAML is missing, so repository verification relied on the full composer dev-tools gate instead.

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-129/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-129/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-129/metrics/