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

packages/opencode/src/config/config.ts

@@ -660,6 +660,10 @@ export namespace Config {
 
   export const Skills = z.object({
     paths: z.array(z.string()).optional().describe("Additional paths to skill folders"),
+    urls: z
+      .array(z.string())
+      .optional()
+      .describe("URLs to fetch skills from (e.g., https://example.com/.well-known/skills/)"),
   })
   export type Skills = z.infer<typeof Skills>