ui: Tree suite with native-fidelity look and ARIA keyboard nav #1037

The core of the library — a tree that looks and behaves like the native VS Code tree. Builds on the @repo/ui scaffold and semantic theme boundary established in DEVEX-618.

Scope

• Tree, TreeItem, TreeGroup, TreeIndentGuide
• useTreeKeyboardNav: roving tabindex, Arrow keys, Home/End, Enter/Space, type-ahead
• ARIA tree roles/attributes, selection model, chevrons, indent guides, hover-revealed trailing action slot (for inline row actions)

Out of scope

• Virtualization (API designed so @tanstack/react-virtual can drop in later without consumer changes)
• Workspace-specific row content

VS Code fidelity

• Match the native VS Code tree look and behavior using native HTML elements styled to match VS Code's own controls, using the VS Code Elements implementation as a design reference without importing it
• Tree row geometry, chevron, indent guide, and selection visuals consistent with VS Code defaults
• Minimize custom styles — rely on inherited values from the WebViews wherever possible; component CSS is inherit-first and never uses line-height/vertical-padding math
• tokens.css maps VS Code variables to semantic --ui-* tokens — add any new tree-specific tokens here (create the file if it doesn't already exist; conflicts will be resolved later)
• control.css provides the shared flex-centering geometry base — add tree row geometry here if applicable (create the file if it doesn't already exist; conflicts will be resolved later)
• Support high contrast through VS Code contrast variables, plus forced-colors and prefers-reduced-motion media handling

Library boundary (NPM-ready)

• No @repo/* or relative cross-package imports from packages/ui (enforced by ESLint)
• React stays a peer dependency; no new runtime dependencies for the tree itself
• Shared internals reached through package.json subpath imports

Storybook

• One .stories.tsx per public component (Tree, TreeItem, TreeGroup, TreeIndentGuide) with a combined matrix story covering nesting, groups, selection, disabled, and action slot
• Each matrix story snapshotted in Light, Dark, High Contrast, and High Contrast Light via Chromatic
• Interaction assertions via play functions; detailed behavior stays in Vitest

Acceptance

• Side-by-side eyeball check against the native tree across the four theme families
• Focused Testing Library/Vitest tests covering keyboard nav behaviors (roving tabindex, Arrow keys, Home/End, Enter/Space, type-ahead)
• Validated with type checking, linting, formatting, UI tests, Storybook CI build, production build, and git diff --check