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/models/session.py

@@ -48,6 +48,9 @@ class Session(Base):
     ticket_number: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
     client_name: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
     exported: Mapped[bool] = mapped_column(Boolean, default=False)
+    session_variables: Mapped[dict[str, Any]] = mapped_column(
+        JSONB, nullable=False, server_default=sa.text("'{}'::jsonb")
+    )
     scratchpad: Mapped[Optional[str]] = mapped_column(
         Text, nullable=True, server_default=sa.text("''")
     )