feat: add procedural flows with intake forms, navigation, and seed templates

Adds a new "procedural" tree type for linear step-by-step project workflows
(domain controller setup, M365 onboarding, VPN config, etc). Includes intake
form builder, two-panel step navigation, variable resolution, procedural
exports, 3 seed templates, and UI rename from "Trees" to "Flows".

Also archives 19 implemented plan docs and creates deferred features backlog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/sessions.py

@@ -136,13 +136,27 @@ async def start_session(
             detail="You don't have access to this tree"
         )
 
+    # For procedural trees with intake forms, validate required fields
+    session_variables = session_data.session_variables or {}
+    if tree.tree_type == 'procedural' and tree.intake_form:
+        missing_fields = []
+        for field in tree.intake_form:
+            if field.get("required") and not session_variables.get(field["variable_name"]):
+                missing_fields.append(field["label"])
+        if missing_fields:
+            raise HTTPException(
+                status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+                detail=f"Missing required intake form fields: {', '.join(missing_fields)}"
+            )
+
     # Create session with tree snapshot (includes tree metadata for filtering/export)
     tree_snapshot = {
         **tree.tree_structure,
         "name": tree.name,
         "description": tree.description,
         "category": tree.category,
-        "version": tree.version
+        "version": tree.version,
+        "tree_type": tree.tree_type,
     }
 
     new_session = Session(
@@ -152,7 +166,8 @@ async def start_session(
         path_taken=[],
         decisions=[],
         ticket_number=session_data.ticket_number,
-        client_name=session_data.client_name
+        client_name=session_data.client_name,
+        session_variables=session_variables,
     )
 
     # Increment tree usage count
@@ -421,7 +436,9 @@ async def save_session_as_tree(
         can_publish, validation_errors = can_publish_tree(
             tree_structure,
             tree_name,
-            request_data.description
+            request_data.description,
+            tree_type=original_tree.tree_type,
+            intake_form=original_tree.intake_form,
         )
         if not can_publish:
             raise HTTPException(