feat: metrickit_reader.py — MetricKit diagnostic payload reader

[conorluddy]

Problem

Apps that adopt MetricKit produce rich JSON diagnostics — hangs, crashes, CPU exceptions, disk write exceptions — but those MXMetricPayload files get lost in the sandbox.

Sketch

python scripts/metrickit_reader.py --bundle-id com.example.app --list
python scripts/metrickit_reader.py --bundle-id com.example.app --show 2026-05-22
python scripts/metrickit_reader.py --bundle-id com.example.app --aggregate 30d

Locates MXMetricPayload-*.json and MXDiagnosticPayload-*.json in the app sandbox via container.py (B1), summarises hang counts, crash counts, top symbols, exception breakdown.

Reuses

• Depends on B1 (container.py) — coordinate ordering. Ship B1 first.
• common/cache_utils.ProgressiveCache for raw payload archive

Open questions

• Confirm MetricKit emits payloads on simulator — historically it was device-only via background delivery. If simulator-blocked, script becomes "device-only" with clear messaging.
• Whether to fall back to scanning the local file system for exported payloads.

Definition of done

• --list, --show, --aggregate working
• Clear error message if simulator doesn't emit
• --json, --help, SKILL.md updated

Part of the v2.0 coverage expansion epic.