Released-docset gate: membership check against the published tags

`docsets.yaml` marks 12 historical docsets (v0.8.1 through v0.15.2) as `released` although no public non-prerelease GitHub release exists for those tags. The released-docset gate therefore compares by order only, as its docstring states: a docset claiming `released` ahead of the newest published release fails, and lag is the normal state after publication.

Once the availability of those 12 entries is decided (left as is on 2026-09-06), add a membership check against the published release list and, if the data is corrected, replace the order rule with the literal "unpublished tag" rule.

Origin: review of PR #850.

Start with docsets.yaml and the released-docset gate's docstring, then inspect how the published GitHub release list is obtained. Confirm the availability decision for the 12 historical docsets before changing the check. Done means the gate checks tag membership literally when the data is corrected, with tests covering unpublished tags.

Written by the indexing model from the issue text.