feat(ui): add accessible VS Code-parity Tree

[EhabY]

Summary

Adds a controlled, accessible Tree component to @repo/ui with current VS Code workbench behavior and visuals.

• Flat, data-driven row projection with aria-activedescendant
• Independent focus and selection, including hidden selection behavior
• Single and multi-selection, ranges, modifier selection, and scoped select-all
• Arrow, Home, End, PageUp, PageDown, Escape, and buffered type navigation
• Single-click and double-click expansion modes
• Focusable but non-selectable disabled rows
• Sticky scroll with pinned-row pointer, keyboard, accessibility, and native-style shadow behavior
• Explorer styling, indent guides, actions, high-contrast support, and Storybook coverage

Closes #1037

Architecture

flowchart LR
  accTitle: Tree architecture
  accDescr: Data and input flow through pure Tree modules into the React and DOM adapter.

  Props[Nodes and controlled props] --> Model[treeModel.ts]
  Events[Pointer and keyboard events] --> Policy[treePolicy.ts]
  Policy --> Commands[Tree commands]
  Model --> Transition[treeTransition.ts]
  Commands --> Transition
  Transition --> Adapter[useTreeAdapter.ts]
  Adapter --> Rows[Tree.tsx and TreeRow.tsx]
  Adapter --> Sticky[StickyScroll.tsx]

Loading

treeModel.ts, treePolicy.ts, and treeTransition.ts are pure. useTreeAdapter.ts is the thin React and DOM boundary. Rendering stays split between normal rows and sticky behavior rather than placing all state and behavior in one component.

The flat visible model supports future windowing, but this Tree is not currently virtualized.

Diff composition

Measured against origin/main...HEAD with git diff --numstat:

Area	Added	Deleted
Production UI source	1,832	0
Automated tests and shared test helpers	1,734	0
Stories and visual fixtures	348	1
Documentation	115	8
Build and workspace configuration	31	2
Total	4,060	11

Production source to automated test code is approximately 1.06:1. Counting stories and visual fixtures as test support gives 1,832 production lines to 2,082 test and visual verification lines.

Performance

Measured in development-mode jsdom against the first data-driven architecture at d794977:

• Keyboard navigation wall time improved by approximately 44% to 57%
• Selection changes improved by approximately 21% to 31%
• Expansion improved at small and medium sizes and was approximately 6% slower at 2,000 rows

These are relative development-mode measurements, not browser frame-time guarantees.

Validation

• pnpm test:webview --reporter=dot: 45 files, 381 tests passed
• Focused Tree suite: 10 files, 63 tests passed
• pnpm typecheck
• pnpm format:check
• pnpm storybook:ci
• git diff --check

Implementation plan and decisions

1. Keep the model, input policy, and interaction transitions pure.
2. Keep React state, controlled callbacks, refs, event normalization, and DOM effects in one thin adapter.
3. Render a flat visible row projection while retaining complete hierarchy data for hidden focus and selection behavior.
4. Preserve VS Code selection, expansion, keyboard, accessibility, sticky-scroll, and visual behavior.
5. Replace the external mutable store and context layers unless measurements justify them.
6. Document the architecture with a compact Mermaid diagram.
7. Split the large component test into focused core, row, selection, keyboard, and sticky suites with shared helpers.
8. Validate behavior, type safety, formatting, linting, builds, Storybook, test counts, and measured interaction performance.

_{This pull request description was updated by Coder Agents on behalf of @EhabY.}