epic: fleet orchestration v2 - staged, nightly-gated, selectively-runnable

[joshua-temple]

Restructure the validation fleet to end the concurrent-token transient flakiness, move release cadence onto a green nightly, and add selective single-member runs. Design: ~/llm/plans/cascade/2026-06-26-fleet-orchestration-v2-plan.md (local). Children below; closed as each is verified on a real fleet run.

• Stage the fan-out (4env solo, primary cluster, remainder max-parallel 2) + repos selector
• Nightly-gated rc-cutting and promotion with change-skip and a workflow_dispatch test vector
• New short-path repo proving the repository_dispatch rollback live (closes test(fleet): run the rollback dispatch, dry-run, and deployable-scoped variants live #321)
• Docs: staged, nightly-gated, selective orchestration