skills: rename pkgrepo -> f3s-pkgrepo and rocky-vm-setup -> f3s-rocky-vm-setup

Extend the f3s- prefix to the two remaining f3s-scoped siblings so the whole
homelab family groups together. Both are f3s-specific: pkgrepo IS
pkgrepo.f3s.buetow.org, and rocky-vm-setup documents the 'rocky' VM on the f3s
host f3.

- git mv both skill dirs (history preserved); update name: frontmatter; add the
  'Part of the f3s homelab skill family' note to both descriptions.
- Rewrite every reference to the *skills*: backtick mentions (`pkgrepo` ->
  `f3s-pkgrepo`, `rocky-vm-setup` -> `f3s-rocky-vm-setup`), markdown cross-links
  (../pkgrepo/ -> ../f3s-pkgrepo/, etc.), the f3s hub Related-skills block and
  description, and skill-maintenance's index-pattern example.
- Left references to the package REPOSITORY (pkgrepo.f3s.buetow.org, 'the custom
  pkgrepo') unchanged — those name a service, not the skill.
- Verified all 114 relative links resolve. Plan doc updated to record the rename.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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