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>

backend/app/schemas/tree_markdown.py

@@ -0,0 +1,28 @@
+"""Pydantic schemas for tree markdown import/export."""
+from pydantic import BaseModel
+
+
+class TreeMarkdownExportResponse(BaseModel):
+    """Response for markdown export endpoint."""
+    markdown: str
+
+
+class TreeMarkdownImportRequest(BaseModel):
+    """Request body for markdown import endpoint."""
+    markdown: str
+
+
+class MarkdownValidationError(BaseModel):
+    """A single validation error with location info."""
+    line: int
+    column: int
+    message: str
+    severity: str  # 'error' or 'warning'
+
+
+class TreeMarkdownValidationResponse(BaseModel):
+    """Response for markdown validation endpoint."""
+    valid: bool
+    errors: list[MarkdownValidationError]
+    tree_structure: dict | None = None
+    metadata: dict | None = None