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/api/router.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter
-from app.api.endpoints import auth, trees, sessions, invite, categories, tags, folders, step_categories, steps, admin, accounts, webhooks, shares, shared
+from app.api.endpoints import auth, trees, sessions, invite, categories, tags, folders, step_categories, steps, admin, accounts, webhooks, shares, shared, tree_markdown
 from app.api.endpoints import admin_dashboard, admin_audit, admin_plan_limits, admin_feature_flags, admin_settings, admin_categories
 
 api_router = APIRouter()
@@ -24,3 +24,4 @@ api_router.include_router(accounts.router)
 api_router.include_router(webhooks.router)
 api_router.include_router(shares.router)
 api_router.include_router(shared.router)  # Public endpoints (no auth)
+api_router.include_router(tree_markdown.router)