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/skill/skill.ts

@@ -11,6 +11,7 @@ import { Filesystem } from "@/util/filesystem"
 import { Flag } from "@/flag/flag"
 import { Bus } from "@/bus"
 import { Session } from "@/session"
+import { Discovery } from "./discovery"
 
 export namespace Skill {
   const log = Log.create({ service: "skill" })
@@ -151,6 +152,22 @@ export namespace Skill {
       }
     }
 
+    // Download and load skills from URLs
+    for (const skillUrl of config.skills?.urls ?? []) {
+      const downloadedDirs = await Discovery.pull(skillUrl)
+      for (const dir of downloadedDirs) {
+        dirs.add(dir)
+        for await (const match of SKILL_GLOB.scan({
+          cwd: dir,
+          absolute: true,
+          onlyFiles: true,
+          followSymlinks: true,
+        })) {
+          await addSkill(match)
+        }
+      }
+    }
+
     return {
       skills,
       dirs: Array.from(dirs),