feat: indicate which skills failed to update, fix #362, code cleanup

[EDM115]

• skills which failed to update/check for update are now properly listed so we can check if the skill still exists or if it have been renamed for instance. example output :

  $ skills check -g
  Checking for skill updates...
  
  Checking 66 skill(s) for updates...
  
  2 update(s) available:
  
    ↑ make-repo-contribution
      source: github/awesome-copilot
    ↑ vueuse
      source: onmax/nuxt-skills
  
  Run npx skills update to update all skills
  
  Could not check 1 skill(s) (may need reinstall)
  
    ✗ vue-development-guides
      source: vuejs-ai/skills
  
  

• when the skills CLI have been installed globally using PNPM for example, update won't work since it calls a nested npx for a package that haven't been retrieved before. skills update now just reuse its own CLI entrypoint to run that. fix [Bug]: The update command can fail when skills is not installed with npm #362
• remove unused consts/functions/types/imports
• fixed an issue with a potentially undefined variable
• refresh the AI instructions
• fixed the link to supported agents in the readme
• format MD files
• prettier scripts now also run fine on Windows

[vercel]

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[quuu]

please resolve merge conflicts, then i'll merge

[EDM115]

good catch for the entrypoint existence 🫡
i'll resolve the conflicts now