When a user pins oh-my-opencode to a specific version (e.g., oh-my-opencode@3.4.0),
the auto-update checker now respects that choice and only shows a notification toast
instead of overwriting the pinned version with latest.
- Skip updatePinnedVersion() when pluginInfo.isPinned is true
- Show update-available toast only (notification, no modification)
- Added comprehensive tests for pinned/unpinned/autoUpdate scenarios
Fixes#1745
