(sidebar): Open always opens the conversation's head; fix the glossary row entry (follow-up #564) #918

Follow-up of #564 (a row is one conversation).

Decided by Jean-Baptiste in person on 2026-10-30: opening a row always opens the newest part (the head), never a dormant part.

Today (measured on main c676f17, .work-files/switchboard/m4-row-definitions.md): a dormant part that is not the head can be opened instead of the head (public/app.js ~2522-2525). Make Open always target headSessionId, with a test for a conversation whose dormant part is older than the head. CHANGELOG entry required if the user can see the difference.

Also fix docs/product/GLOSSARY.md ~108, which still says no doc defines what a row represents: point it to docs/product/CONCEPT-MAP.md §1 (a row is one conversation, decided in #564).