feat(check): an offline input runs the mirror's offline verb

[jshvn]

Consistency review C7, lib part (PR B of lib v2.6.0).

• check.yml gains a boolean input offline (default false). When it is true, the job runs task run -- task offline after task check.
• README, ### check.yml: a mirror with an offline verb sets offline: true; it no longer needs its own job.

A caller that passes no inputs (ctan, tlnet, gnu, nongnu) skips the new step, so nothing changes for them. github (phase 3) and dropbox (phase 4) become 6-line callers with offline: true.

Test plan

• actionlint on base and head: same single pre-existing finding (github.job_workflow_sha is missing from actionlint's context table); a misspelled inputs.offlin is caught, so inputs.offline is valid
• Callers with no with: keep the default (checked ctan, tlnet, gnu, nongnu)
• First live offline: true run: github's phase 3 caller, pinned to v2.6.0