fix: auto-migrate Windows config from legacy path

Previously, when config existed at legacy %APPDATA%\opencode path,
the plugin would read from there but never move it - leaving the
config permanently misaligned with the standard ~/.config/opencode path.

Now the plugin auto-migrates:
1. Attempts rename() first (atomic, same filesystem)
2. Falls back to copy()+delete() for cross-filesystem scenarios
3. Only uses legacy path as fallback if migration fails

This aligns config location across all platforms after first use.