Range-diff !482

back to !482 ci: reserve ci/ statuses; trusted reuse; required contexts

 1:  848f79b =  1:  848f79b runner next: say whether the build is trusted
 2:  5f01597 =  2:  5f01597 runner: disposable home for untrusted builds
 3:  bb4a146 =  3:  bb4a146 wiki: trusted and untrusted build homes
 4:  2e830df =  4:  2e830df status set: ci/ is reserved for the instance's builds
 5:  3453d63 =  5:  3453d63 ci: reuse only trusted results on the job's image
 6:  bba63a3 =  6:  bba63a3 repo settings: required contexts turn the checks gate on, pending until reported
 7:  60cf9c5 =  7:  60cf9c5 wiki: reserved ci/ statuses, trusted reuse, required contexts
 8:  734d0a4 =  8:  734d0a4 changelog: #255, #258
 -:  ------- >  9:  ef74a96 wiki, changelog: last finisher sets ci/<job>; required contexts report on heads