chore(ci): replace mirror.yml with reusable wrapper #58

Job	Run time
governance / Guix primary / Nix fallback policy	45s
governance / Code quality + docs	38s
governance / Validate Hypatia baseline	33s
governance / Workflow security linter	32s
governance / Language / package anti-pattern policy	34s
governance / Well-Known (RFC 9116 + RSR)	38s
governance / Security policy checks	41s
	4m 21s

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