ci: route cmux-tui artifact jobs through live fleet
The cmux-tui artifact tree publisher could queue its Linux control and daemon jobs on a stale Blacksmith 4-vCPU label while the live fleet was unavailable. That leaves the tree manifest and daemon marker missing, so nightly-next cannot consume an otherwise built source.
This change carries the approved live GCP runner-group reader and adds a Linux-only placement step to tree preflight. It emits cmux-fleet-linux-x64 only when the authenticated runner inventory proves an online member; otherwise the daemon and other heavy work use measured Blacksmith overflow. The reusable package workflow receives that live output, and stale Blacksmith values cannot pin the selector. The picker reuses the workflow's existing routing App token under its existing builder-gated condition, with its existing administration/read and organization-self-hosted-runners/read permissions; no new App, secret, or permission is introduced. The active owner run 41289457009 is left untouched.
Validation:
python3 -m unittest tests.test_ci_portable_job_placement tests.test_cmux_tui_artifacts_linux_runner_set tests.test_ci_pr_runner_pool.OwnedPools(86 passed)- YAML parsing, Python compilation and
git diff --checkpass - The broader failover suites still report only their known base reds in
cmux-next-chromium-currency.ymlandplatform-dev-secrets.yml
Agent session: agent session