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/session/VariablePromptModal.tsx

@@ -0,0 +1,73 @@
+import { useState } from 'react'
+import { cn } from '@/lib/utils'
+
+interface VariablePromptModalProps {
+  /** The prompt text from [USER_INPUT:prompt] */
+  prompt: string
+  /** Called with the user's input value */
+  onSubmit: (value: string) => void
+  /** Called when user cancels */
+  onCancel: () => void
+}
+
+export function VariablePromptModal({ prompt, onSubmit, onCancel }: VariablePromptModalProps) {
+  const [value, setValue] = useState('')
+
+  const handleSubmit = (e: React.FormEvent) => {
+    e.preventDefault()
+    if (value.trim()) {
+      onSubmit(value.trim())
+    }
+  }
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm">
+      <div className="glass-card w-full max-w-md rounded-2xl p-6 shadow-lg">
+        <h2 className="mb-1 text-lg font-semibold text-white">Input Required</h2>
+        <p className="mb-4 text-sm text-white/40">
+          This step requires you to provide a value.
+        </p>
+
+        <form onSubmit={handleSubmit}>
+          <label className="mb-2 block text-sm font-medium text-white/70">
+            {prompt}
+          </label>
+          <input
+            type="text"
+            value={value}
+            onChange={(e) => setValue(e.target.value)}
+            placeholder="Enter value..."
+            autoFocus
+            className={cn(
+              'w-full rounded-lg border border-white/10 bg-black/50 px-3 py-2 text-sm text-white',
+              'placeholder:text-white/30 focus:border-white/30 focus:outline-none focus:ring-1 focus:ring-white/20'
+            )}
+          />
+
+          <div className="mt-4 flex gap-2">
+            <button
+              type="submit"
+              disabled={!value.trim()}
+              className={cn(
+                'flex-1 rounded-lg bg-white px-4 py-2 text-sm font-medium text-black',
+                'hover:bg-white/90 disabled:opacity-50 disabled:cursor-not-allowed'
+              )}
+            >
+              Continue
+            </button>
+            <button
+              type="button"
+              onClick={onCancel}
+              className={cn(
+                'rounded-lg border border-white/10 px-4 py-2 text-sm font-medium text-white/60',
+                'hover:bg-white/10 hover:text-white'
+              )}
+            >
+              Skip
+            </button>
+          </div>
+        </form>
+      </div>
+    </div>
+  )
+}