P5: New benchmark-tier workflow — runs after the weekly soak, own ≤24h budget

[FumingPower3925]

Part of #151.

Goal. A scheduled benchmark that runs right after the weekend soak, on the cluster, within its own ≤24h window.

Current state. No workflow runs mage Bench on a schedule — nightly/weekend run mage Validate/mage Soak only. Bench is manual.

Design.

• New benchmark-tier.yml, triggered by workflow_run on matrix-weekend-tier completion (runs 'after the soak'), in the shared concurrency: matrix-tier-cluster group with cancel-in-progress: false — queues behind the soak, never overlaps.
• Reuse the setup → matrix → teardown skeleton (cluster-runner-up/down). Provision fresh runners for the bench run so budgets are independent.
• Keep workflow_dispatch for manual kicks. (Scheduled run is maintainer-owned, so the PR-tier label gate isn't needed here.)
• Its own ≤24h budget (separate from the soak's 24h).

Acceptance. After a weekend soak completes, the benchmark workflow auto-starts, runs the curated matrix on both arches, publishes (per the publish-pipeline issue), tears down cleanly.

Refs: .github/workflows/matrix-weekend-tier.yml, .github/actions/cluster-runner-{up,down}, mage_bench.go, ansible/bench.yml.