Companion to sow-platform#189 (runner capacity 1 -> 2). Capacity 1 was the implicit mutex serializing every workflow; this makes the exclusions explicit: side-effectful workflows serialized (never cancelled mid-run), PR-triggered checks per-ref with cancel-in-progress.Reviewed-on: #102 Co-authored-by: vickydotbat <vickydotbat@tutamail.com>