Skills from skills.urls are cached by existence and never updated #33548

Skills pulled from skills.urls are cached on disk and never refreshed, so an updated skill never reaches users who already pulled it once.

In packages/opencode/src/skill/discovery.ts the per-file download short-circuits on existence:

const download = Effect.fn("Discovery.download")(function* (url, dest) {
  if (yield* fs.exists(dest).pipe(Effect.orDie)) return true
  // ... fetch + write
})

index.json is re-fetched on every run, but it's only used to enumerate files — nothing compares the remote content against what's already in ~/.cache/opencode/skills/<name>/. Once SKILL.md exists locally, any change published at the URL is ignored.

Repro:

1. Host a skill via skills.urls, start opencode (it caches SKILL.md).
2. Edit and re-publish the same skill at the same URL.
3. Restart opencode — the stale SKILL.md is still used.

The only workaround is having every user rm -rf ~/.cache/opencode/skills/<name>. That doesn't scale when you maintain a shared catalog and ship fixes.

Proposal: make the cache version-aware. index.json already round-trips on every run, so an optional version (or content hash/etag) per skill entry is enough — on pull, if the cached version differs from the index, re-download that skill's files. No version present → current behavior, so it stays backward compatible.

Happy to send a PR if this direction is acceptable.