plan-eng-review: add throughput planning readout

[al-buda-noygear]

Summary

• add a required Throughput planning readout section to plan-eng-review
• make every eng review emit a strict build stackrank, max safe parallelism, expected wall clock, and practical total effort
• carry the same requirement into the skill template so the prompt stays durable

Why

For implementation planning, the scarcest resource is usually not code generation but human attention: deciding what blocks what, where coordination is required, and how much work can safely run in parallel.

The existing eng review already covers architecture, tests, failure modes, and worktree parallelization, but it does not force a final execution readout that answers:

• what should happen first
• what can happen at the same time
• where human attention is actually required
• how long the accepted scope will really take
• how many agent can run in parallel (minimize dependencies, maximize parallelism)
• what each agent should execute on in what order (execution map when dependencies do exist)

This change makes that throughput framing a first-class part of every eng review.