chore: adopt reusable FloowGitHub skill

[Tutitoos]

Related issue

Closes #159

Result

• ATENEA now references the installed $floowgithub skill.
• Repository-specific policy is retained in .github/floowgithub.json.
• The duplicated repository-local delivery skill is removed.
• ATENEA CI and v1-readiness now validate the repository-owned FloowGitHub policy instead of the retired local copy.

Validation and evidence

• FloowGitHub repository CI passed for published SHA e74f3d68336574d728a3eb4c35ce646cde047410.
• Installed the private GitHub skill into ~/.codex/skills/floowgithub and validated its structure.
• New FloowGitHub behavioral suite: 13/13 passed.
• Historical ATENEA delivery fixtures against FloowGitHub: 27/27 preserved their readiness result.
• ATENEA policy snapshot validation passed.
• Hugo documentation build passed locally with 57 pages.
• ATENEA commit and pre-push hooks passed, including the full race suite required by the hook.
• All GitHub-hosted checks passed for head 12ec7c4f068a26a0e891d37fc81ea4036375c20a; opt-in jobs were skipped as designed.
• The macOS Intel timing test failed once and passed on the single permitted unchanged-SHA retry.

Unrun checks

None required for this migration.

Risks and rollback

ATENEA now depends on access to the private Tutitoos/floowgithub repository for fresh installations. Roll back these commits to restore the repository-local copy.