chore(gitignore): ignore dune-generated `*.install` manifests (#306) #255

Job	Run time
governance / Code quality + docs	10s
governance / Workflow security linter	6s
governance / Security policy checks	4s
governance / Language / package anti-pattern policy	6s
governance / Guix primary / Nix fallback policy	6s
governance / Well-Known (RFC 9116 + RSR)	4s
	36s

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