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/components/tree-editor/code-mode/resolutionFlowLanguage.ts

@@ -0,0 +1,72 @@
+import type { languages } from 'monaco-editor'
+
+/**
+ * Monarch tokenizer for ResolutionFlow tree markdown syntax.
+ * Provides syntax highlighting for frontmatter, headings, options, references, and variables.
+ */
+export const resolutionFlowLanguage: languages.IMonarchLanguage = {
+  tokenizer: {
+    root: [
+      // Frontmatter delimiter
+      [/^---\s*$/, 'delimiter.frontmatter', '@frontmatter'],
+
+      // Headings
+      [/^##\s+.*$/, 'heading.action'],
+      [/^#\s+.*$/, 'heading.decision'],
+
+      // Blockquote (help text)
+      [/^>\s.*$/, 'string.blockquote'],
+
+      // Option lines: - [X] Label → @target_id
+      [/^-\s*\[[A-Za-z0-9]+\]/, 'keyword.option', '@optionLine'],
+
+      // Next node reference: → @node_id
+      [/^→\s*@\S+/, 'variable.reference'],
+
+      // Expected outcome
+      [/^\*\*Expected:\*\*\s*.*$/, 'keyword.expected'],
+
+      // Command block
+      [/^```commands\s*$/, 'delimiter.commands', '@commandBlock'],
+      [/^```\s*$/, 'delimiter.commands'],
+
+      // Variable tokens
+      [/\[USER_INPUT:[^\]]+\]/, 'variable.input'],
+      [/\[VAR:[^\]]+\]/, 'variable.reference'],
+      [/\[SAVE_AS:[^\]]+\]/, 'variable.save'],
+
+      // Ordered list items
+      [/^\d+\.\s+/, 'keyword.step'],
+
+      // Bold
+      [/\*\*[^*]+\*\*/, 'strong'],
+
+      // Inline code
+      [/`[^`]+`/, 'string.code'],
+
+      // Regular text
+      [/./, 'text'],
+    ],
+
+    frontmatter: [
+      [/^---\s*$/, 'delimiter.frontmatter', '@pop'],
+      [/^(id)(:)(.*)$/, ['keyword.frontmatter', 'delimiter', 'string.id']],
+      [/^(type)(:)(.*)$/, ['keyword.frontmatter', 'delimiter', 'type.value']],
+      [/^(parent)(:)(.*)$/, ['keyword.frontmatter', 'delimiter', 'string.parent']],
+      [/./, 'variable.other'],
+    ],
+
+    optionLine: [
+      [/→\s*@\S+/, 'variable.reference'],
+      [/$/, '', '@pop'],
+      [/./, 'string.option'],
+    ],
+
+    commandBlock: [
+      [/^```\s*$/, 'delimiter.commands', '@pop'],
+      [/./, 'string.command'],
+    ],
+  },
+}
+
+export const LANGUAGE_ID = 'resolutionflow'