Investigate migrations via throwaway subagent experiments without modifying tracked files

Before committing to a dependency migration (e.g., Astro 6→7, Vite 7→8), the user dispatches a subagent to a throwaway directory like /tmp/opencode/<name> to resolve and probe the migration surface. The subagent must NOT modify tracked files (package.json, pnpm-workspace.yaml, pnpm-lock.yaml), must report findings as a numbered list, and must clean up after itself. Only after the user reviews the surface map do they apply the actual edits — for example, replacing the vite override rules once the peer-error cascade is understood. Findings should include: resolved target versions, peer-dep cascades, required config edits, and any blocked paths. Follow this pattern when proposing version bumps: preview in throwaway first, then apply.