feat: use @pierre/trees for Studio file tree

[miguel-heygen]

What changed

• replaced the Studio file tree with an @pierre/trees-based wrapper while keeping the existing Studio actions and affordances wired through it
• preserved create, rename, delete, duplicate, import targeting, root and row context menus, selection syncing, and empty-folder .gitkeep handling
• added focused tests for path shaping, drop targeting, move destination building, and pending-create lifecycle behavior

Why

Studio needed the richer tree viewer from @pierre/trees, but the first integration attempt exposed a wrapper bug: inline renames for newly created items could lose pending-create state and fall through to a rename request instead of the Studio create API. This patch fixes that race so new files and folders persist correctly after the inline rename flow.

Impact

• Studio gets the new tree viewer library without regressing the existing file operations UX
• newly created files and folders now reliably hit the correct Studio backend APIs during inline rename

Validation

• bunx oxfmt packages/studio/src/App.tsx packages/studio/src/components/editor/FileTree.tsx packages/studio/src/components/editor/FileTree.test.ts
• bunx oxlint packages/studio/src/App.tsx packages/studio/src/components/editor/FileTree.tsx packages/studio/src/components/editor/FileTree.test.ts
• bun run --cwd packages/studio test src/components/editor/FileTree.test.ts
• bun run --cwd packages/studio typecheck
• agent-browser browser verification on local Studio for file-create and folder-create inline rename flows, including persistence after reload

Notes

• left an unrelated untracked .thumbnails/ artifact out of the PR scope
• keeping this PR as a draft so drag/drop move behavior can still get a final manual browser pass on GitHub if desired