feat(skill): add skill discovery from URLs via well-known RFC

Implement the Agent Skills Discovery RFC to allow fetching skills from URLs:
- Add 'urls' field to config.skills for specifying skill registry URLs
- Create Discovery namespace in skill/discovery.ts with pull() function
- Download skills from /.well-known/skills/index.json endpoints
- Cache downloaded skills to ~/.cache/opencode/skills/
- Skip re-downloading existing files for efficiency

Users can now configure:
{
  "skills": {
    "urls": ["https://example.com/.well-known/skills/"]
  }
}

Implements: https://github.com/cloudflare/agent-skills-discovery-rfc

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