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/lib/variableResolver.ts

@@ -0,0 +1,49 @@
+/**
+ * Frontend-side variable substitution for display during navigation.
+ *
+ * - [VAR:name] → replaced with variables[name]
+ * - [USER_INPUT:prompt] → replaced with variables[prompt]
+ * - [SAVE_AS:name] → removed from display
+ */
+export function resolveVariables(text: string, variables: Record<string, string>): string {
+  // Replace [VAR:name]
+  let result = text.replace(/\[VAR:([^\]]+)\]/g, (_, name) => {
+    const key = name.trim()
+    return variables[key] ?? `[VAR:${key}]`
+  })
+
+  // Replace [USER_INPUT:prompt]
+  result = result.replace(/\[USER_INPUT:([^\]]+)\]/g, (_, prompt) => {
+    const key = prompt.trim()
+    return variables[key] ?? `[USER_INPUT:${key}]`
+  })
+
+  // Remove [SAVE_AS:name]
+  result = result.replace(/\[SAVE_AS:[^\]]+\]/g, '')
+
+  return result
+}
+
+/**
+ * Extract all [USER_INPUT:prompt] tokens from text.
+ * Returns array of prompt strings.
+ */
+export function extractUserInputPrompts(text: string): string[] {
+  const prompts: string[] = []
+  const re = /\[USER_INPUT:([^\]]+)\]/g
+  let match
+  while ((match = re.exec(text)) !== null) {
+    const prompt = match[1].trim()
+    if (!prompts.includes(prompt)) {
+      prompts.push(prompt)
+    }
+  }
+  return prompts
+}
+
+/**
+ * Check if text contains any variable tokens.
+ */
+export function hasVariableTokens(text: string): boolean {
+  return /\[(USER_INPUT|VAR|SAVE_AS):[^\]]+\]/.test(text)
+}