feat: add dual-mode tree editor with Code Mode, variables, and markdown sync

Implements the full dual-mode tree editor (Plan Phases 1-5):

Backend:
- JSONB↔Markdown bidirectional serializer/parser with mistune
- Markdown validator with line/column error reporting
- 3 API endpoints: export-markdown, import-markdown, validate-markdown
- Variable extraction/resolution service ([USER_INPUT], [VAR], [SAVE_AS])
- Session variables JSONB column (migration 028)
- 39 tree markdown tests + variable service tests (403 total passing)

Frontend:
- Monaco-based Code Mode with custom Monarch tokenizer and dark theme
- Autocomplete for @node_id refs, type values, variable names
- Debounced validation (800ms) with inline Monaco error markers
- Syntax help panel (absolute overlay, toggleable)
- Starter template for new trees with valid cross-references
- Bidirectional metadata sync (name/description/category/tags frontmatter)
- Synchronous tree→markdown serializer (fixes async race condition)
- Pre-save validation blocks save on broken refs or missing tree name
- Mode-aware undo/redo: Monaco native in Code Mode, throttled zundo in Flow Mode
- Variable prompt modal and frontend resolver for session navigation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/types/tree.ts

@@ -185,3 +185,25 @@ export interface TreeValidationResponse {
   can_publish: boolean
   errors: ValidationError[]
 }
+
+// Tree markdown types
+export interface TreeMarkdownValidationError {
+  line: number
+  column: number
+  message: string
+  severity: 'error' | 'warning'
+}
+
+export interface TreeMarkdownMetadata {
+  name?: string
+  description?: string
+  category?: string
+  tags?: string[]
+}
+
+export interface TreeMarkdownValidation {
+  valid: boolean
+  errors: TreeMarkdownValidationError[]
+  tree_structure: TreeStructure | null
+  metadata?: TreeMarkdownMetadata | null
+}