fix(crosscheck): queue shared Azure capacity

[ruby-dlee]

Intent

Prevent transient Azure family exhaustion from turning concurrent Crosscheck reviews into TOOL-FAILURE without buying quota or adding standing capacity.

What changed

• spread four review lanes across four approved VM families
• route transient family pressure through the durable reservation queue
• keep the same reservation identity across waits and restarts
• bound timeouts and clean up on timeout or non-capacity refusal
• revalidate credentials after capacity admission
• remove the live reviewer SKU pin from the default operating contract

Validation

• focused Azure Crosscheck behavioral suite passed
• full repository behavior suite passed
• agent-fleet Python suite: 451 passed
• shell lint, Python compile, and diff checks passed
• independent exact-head Azure Crosscheck remains the merge gate