Retired Linear automatic processing

The Linear-label agent poller and host trigger watcher were removed on September 7, 2026 (TASK-7543). Labels no longer schedule or launch coding chats. Do not reinstall their services.

scripts/linear-cron-setup.sh retains only the Linear archive, artifact cleanup and legacy session cleanup services. _linear_client.py, _zellij_utils.py, existing issue history and scripts/.tmp/poller-sessions.json are retained for manual tooling and compatibility. No historical task/worktree records are deleted by this removal.

TASK-8338 owns a future review of useful event-triggered investigations as OpenMates workflows, due September 14. No replacement scheduler is introduced here. The new openmates-ci-coordinator is independent and remains intact.