Fix nested skill discovery for skills.sh installs

[hungpixi]

This marks the repo-level wrapper skill and the template skill as internal so npx skills add Affitor/affiliate-skills discovers the actual nested skills by default.

Why this helps:

• today the repo root has a SKILL.md, so the skills CLI treats the repo as a single root skill and stops early
• that means nested skills under skills/<stage>/<skill>/SKILL.md are skipped unless the user passes --full-depth
• some agent-specific mappings in nested agents/openai.yaml only become reachable when those real skills are discovered

What changed:

• SKILL.md: add metadata.internal: true
• template/SKILL.md: add metadata.internal: true

Result:

• default discovery now skips the root wrapper/template and finds the real published skills
• users no longer need --full-depth just to install the full skill set

Verification:

• before: npx skills add Affitor/affiliate-skills --yes --list surfaced only the root skill
• after this patch (tested against the fork locally): npx skills add D:/code/affiliate-skills --yes --list surfaces 52 skills without --full-depth