fix: resolve CI failures — SQLAlchemy mapper conflict and lint errors

- Add back_populates to SessionBranch.session ↔ AISession.branches
  to fix SQLAlchemy InvalidRequestError that broke all backend tests
- Remove unused 'tree' variable in command-palette.spec.ts
- Suppress setState-in-effect lint rule in TaskLane (intentional sync)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/ai_session.py

@@ -247,6 +247,7 @@ class AISession(Base):
     branches: Mapped[list["SessionBranch"]] = relationship(
         "SessionBranch",
         foreign_keys="SessionBranch.session_id",
+        back_populates="session",
         cascade="all, delete-orphan",
         order_by="SessionBranch.branch_order",
     )