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

[thdxr]

Summary

This PR implements the Agent Skills Discovery RFC, allowing users to specify URLs for skill registries in their config.

Changes

• src/config/config.ts: Added "urls" field to "skills" schema
• src/skill/discovery.ts (new): Created "Discovery" namespace for downloading skills from well-known endpoints
• src/skill/skill.ts: Integrated discovery to load skills from configured URLs

Usage

Users can now add skill registry URLs to their config:

"""json
{
"skills": {
"urls": ["https://example.com/.well-known/skills/"]
}
}
"""

How it works

1. Fetches "/.well-known/skills/index.json" from each URL
2. Downloads all skill files listed in the index
3. Caches them to "~/.cache/opencode/skills/"
4. Skips re-downloading existing files for efficiency
5. Loads the downloaded skills into the skill registry

Implementation details

• Follows the naming conventions from @AGENTS.md
• Caches files to avoid re-downloading on every run
• Returns empty array on failure (non-blocking)
• Logs progress and errors appropriately

Testing

• Typecheck passes ✓
• All existing tests pass ✓
• Implementation follows RFC spec exactly