Implement Tree Editor with visual preview and documentation updates

Tree Editor Features:
- Zustand store with immer middleware and zundo for undo/redo
- Form-based node editing (Decision, Action, Solution types)
- Visual tree preview with solution connection indicators
- NodePicker with type-grouped dropdown (Decisions/Actions/Solutions)
- SharedLinksMap for detecting nodes with multiple sources
- Modal component with scrollable body, fixed header/footer

New Components:
- TreeEditorLayout, TreeMetadataForm, NodeList, NodeEditorModal
- NodeFormDecision, NodeFormAction, NodeFormResolution
- DynamicArrayField, NodePicker
- TreePreviewPanel, TreePreviewNode

Documentation:
- Updated README.md status to Phase 2
- Added Tree Editor details to CURRENT-STATE.md
- Added modal/Zustand lessons to LESSONS-LEARNED.md
- Updated file structure in CLAUDE-SETUP.md
- Added Tree Editor progress to PROGRESS.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/tree-editor/index.ts

@@ -0,0 +1,9 @@
+export { TreeEditorLayout } from './TreeEditorLayout'
+export { TreeMetadataForm } from './TreeMetadataForm'
+export { NodeList } from './NodeList'
+export { NodeEditorModal } from './NodeEditorModal'
+export { NodeFormDecision } from './NodeFormDecision'
+export { NodeFormAction } from './NodeFormAction'
+export { NodeFormResolution } from './NodeFormResolution'
+export { DynamicArrayField } from './DynamicArrayField'
+export { NodePicker } from './NodePicker'