docs(skills): document mandatory version pinning and version: latest

[albertodebortoli]

Summary

• Documents the versioned skill cache paths and repo-level version inheritance introduced in Luca#99: version is now required on every skill entry (or inherited from a repos: alias), skills cache at ~/.luca/skills/{name}/{version}/, and the same repository can be referenced at multiple versions without conflict.
• Documents version: latest / --ref latest from Luca#101, including the network round-trip and reproducibility tradeoffs of using latest.
• Fixes previously-outdated Lucafile/CLI examples across the skills docs that omitted the now-required version/--ref.

Files changed

• docs/skills/lucafile-skills.md — new Versioning + expanded Repository Aliases sections
• docs/skills/installing-skills.md — --ref flag docs, updated luca installed --skills output
• docs/skills/global-skills.md — fixed outdated examples
• docs/getting-started/quick-start.md — added required version: to the Lucafile walkthrough

Test plan

• npm run docs:build passes (also catches dead links)
• Visual check of rendered pages via npm run docs:preview