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",
     )

backend/app/models/session_branch.py

@@ -51,7 +51,7 @@ class SessionBranch(Base):
     updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))
 
     # Relationships
-    session: Mapped["AISession"] = relationship("AISession", foreign_keys=[session_id])
+    session: Mapped["AISession"] = relationship("AISession", foreign_keys=[session_id], back_populates="branches")
     parent_branch: Mapped[Optional["SessionBranch"]] = relationship("SessionBranch", remote_side="SessionBranch.id", foreign_keys=[parent_branch_id])
     fork_point_step: Mapped[Optional["AISessionStep"]] = relationship("AISessionStep", foreign_keys=[fork_point_step_id])
     status_changed_by_user: Mapped[Optional["User"]] = relationship("User", foreign_keys=[status_changed_by])