Add rv-frame-compare registry entry (URL-only)

## Summary - Converts the rv-frame-compare entry to a URL-only registry entry - Removes embedded plugin source code (now lives at https://github.com/erikstrauss/rv-frame-compare) - Updates `index.md` schema: replaces `repoPath` with `repoUrl` and `repoOwner` - Retains `feature.svg` thumbnail This PR depends on #47 (cleanup of old embedded entries) but can be reviewed in parallel. ## Test plan - [ ] Verify `plugins/OpenRV/rv-frame-compare/` contains only `index.md` and `feature.svg` - [ ] Confirm `repoUrl` points at `https://github.com/erikstrauss/rv-frame-compare` - [ ] After merge, confirm the plugin card appears on the registry site within minutes Signed-off-by: erikstrauss <erik.strauss@gmail.com>