Merge branch 'feat/session-branch-selection' into dev

Resolved conflicts:
- Moved branch selection UI from inline sessions.tsx to CreateSessionForm component
- Integrated branch dropdown and new branch creation into CreateSessionForm
- Removed duplicate branch state management from sessions.tsx

All branch selection tests pass (7/7).

apps/web/src/api/tool_types.ts

@@ -21,7 +21,6 @@ export interface ToolType {
   build_context: Record<string, string> | null;
   readiness_probe: ReadinessProbe | null;
   required_variables: string[];
-  is_builtin: boolean;
   created_by_id: string | null;
   created_at: string;
   updated_at: string;