chore(metrics): measure adoption of source-verification and property-based-testing skills (#2172)

[bdfinst]

Part of #2200. Blocked on slice 0.

Measure

• Invocations: count from ~/.claude/metrics/artifact-usage.json via /artifact-lifecycle, split into direct user invocations and dispatches from the wired consumers (doc-review/tech-writer and correctness-review category 6 for source-verification; test-design-advisor for PBT).
• source-verification: the verdict mix (verified / contradicted / unverifiable). A contradicted verdict that caught a real wrong claim is the benefit signal. Report the rate per invocation against slice 0's threshold; one catch over the whole window is not enough to keep the skill.
• property-based-testing: the mutants killed by the generated properties (/mutation-testing) against the same target's pre-existing tests.

Acceptance criteria

• Zero consumer dispatches over the window marks the skill as the orphan pattern feat(plugin): nWave comparison follow-ups — subagent lifecycle hooks and two skill gaps #2172 itself warned about ("adding a skill nothing dispatches"). Recommend either wiring it or archiving it per /artifact-lifecycle's rules.
• Results are posted to chore(metrics): evaluate realized benefits of epics #2164 and #2172 #2200.

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