`preview` is the head of the first user message and the title fallback on
every surface — sidebar rows, pickers, exports, the desktop's sessionTitle().
An untitled /skill session therefore read `[IMPORTANT: The user has invoked
the "work" skill, indicatin...` wherever it appeared.
A scaffolded row now selects a wide enough excerpt to reach the typed
instruction (head + tail spliced for a long body) and shapes it through
describe_skill_invocation(). Because previews are computed on read, existing
sessions are corrected without a migration.
The six copies of the preview subquery and four copies of its shaping collapse
into one expression and one helper along the way, and the /rewind picker gets
the same treatment.
