Review fixes: recheck isUserInvocable on the loaded definition (list and
get collect independently, so a provider change between them could swap in
a user-disabled body — the skill-tool execute template's second check);
thread the carrier signal through the lookup and refuse an abandoned
caller's turn as cancelled; fold lookup/loader failures into the
structured internal error the list face already uses; refuse cwd-less
sessions with the skill.list stance; and reject blank trailing text at the
wire schema instead of relying on client trimming.
Yichen Jiang
c4c2355b50