skills: add mz-debug-replica-latency triage skill - #37292
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Triage procedure for production replica stalls and slow-persist-op warnings. Distinguishes backing-store slowness, runtime stalls, shard contention, and memory pressure using Grafana metrics and PolarSignals profiles. References cover the swap-thrash and CPU-starvation branches. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Adds an agent skill for triaging a production replica that stalls or runs slow.
The skill distinguishes the four common root causes of a multi-second internal latency warning, which usually names a downstream symptom rather than the cause: backing-store (CRDB) slowness, a stalled process runtime, shard contention, and local memory pressure.
It walks the decision tree with Grafana (Prometheus) and PolarSignals, using metric names as the stable contract and discovering datasources and identifiers at runtime, so it carries no environment- or customer-specific values.
Reference files cover the swap-thrash and CPU-starvation branches in detail.
Derived from a recent investigation where a
persist reader heartbeat call took 60swarning turned out to be swap thrashing on a single replica, not a consensus problem. See DB-153.