feat(skills): /test-matrix 5-phase pipeline
Adds hub-and-spoke testing-matrix skill complementing /test-gen:
SKILL.md index + phase-1-intake (language/coverage/critical/CI),
phase-2-matrix (test-type × language multi-select), phase-3-scaffold
(config + corpus + fixtures per cell), phase-4-ci-wire (per-type
failure policy + artifacts), phase-5-triage (crash/regression runbook).
Cross-refs _blocks/test-fuzz.md, test-property.md, test-load.md,
test-e2e.md. Adds "complements" note to skills/test-gen/SKILL.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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