Cursor prompt: Add WorkspaceStore module

Prerequisites

Read:

• [ ] [ONTOUI.md](../WORKSPACE_RUNTIME.md`
• [ ] 0004-workspacestore-ui-source-of-truth.md

Non-goals

• WorkspaceRegistry
• Migrate all panels
• Persist state to disk

Current state

• No centralized store; panels use local useState
• messages.ts defines LSP payload types

Tasks

1. Add zustand (or use existing state lib if present) to webview-ui package.json if needed
2. Create extension/webview-ui/src/store/types.ts with WorkspaceStore slice types from WORKSPACE_RUNTIME.md
3. Create extension/webview-ui/src/store/workspaceStore.ts with initial state and selectors
4. Create extension/webview-ui/src/store/index.ts exports
5. Add store/workspaceStore.test.ts with focus/selection update tests

Acceptance criteria

• [ ] Store initializes with null focus
• [ ] Tests cover setFocus and setSelection
• [ ] Types match platform doc

Tests

• cd extension/webview-ui && npm test

Do not

• Do not wire every panel yet
• Do not add event bus until prompt 05

References

• Cursor prompts index