refactor: delegate 'agent skills install' to the skills CLI via npx

[ramya18101]

🔧 Changes

Reworks auth0 agent skills install to delegate to the open agent-skills CLI (vercel-labs/skills, run via npx) instead of the CLI's own bespoke installer.

📚 References

• Skill source repo: https://github.com/auth0/agent-skills

🔬 Testing

-Manual, end-to-end:

• auth0 agent skills install → interactive picker (skills).
• auth0 agent skills install --agent claude-code,cursor → non-interactive, no prompts.
• auth0 agent skills install --no-input / --force → runs prompt-free.
• PATH without npx → clear "requires Node.js" error

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 62.06897% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.90%. Comparing base (86b69ea) to head (7d40f18).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cli/skills.go	62.06%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1619      +/-   ##
==========================================
+ Coverage   60.60%   60.90%   +0.30%     
==========================================
  Files         135      138       +3     
  Lines       26385    26957     +572     
==========================================
+ Hits        15991    16419     +428     
- Misses       9407     9513     +106     
- Partials      987     1025      +38     

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[developerkunal]

LGTM