Re-mirror SDK source closure at the docs cohort head (tbh #26230/#26772/#26724)

Verbatim closure from mslsrc/tbh@507c86fef428fb0eebade068433fdc4e058eed88 per
scripts/sdk-source-closure.json: session-fold noneReason/recovery split
(#26772), excerpt-region fix cohort transcripts (#26230), and the sdk-ts
surface as gate-accepted on that main push (tbh-muse-developer-docs run
33591321609, developer-docs job green). Hand-authored files preserved;
publish-anchor.json advanced to the new anchor. Manual bridge per the
standing rule (MuseCode Release app not installed on this repository).