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/alembic/versions/028_add_session_variables.py

@@ -0,0 +1,37 @@
+"""add session_variables JSONB column to sessions
+
+Revision ID: 028
+Revises: 027
+Create Date: 2026-02-09
+
+Adds session_variables JSONB column for storing variable values
+collected during tree navigation (e.g., hostname, ticket_number).
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+
+# revision identifiers, used by Alembic.
+revision: str = '028'
+down_revision: Union[str, None] = '027'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'sessions',
+        sa.Column(
+            'session_variables',
+            postgresql.JSONB(),
+            server_default=sa.text("'{}'::jsonb"),
+            nullable=False,
+        )
+    )
+
+
+def downgrade() -> None:
+    op.drop_column('sessions', 'session_variables')