fix(host): harden skill.invoke at the enforcement boundary

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.

packages/host/apiproxy/src/fetch/handler.ts

@@ -109,7 +109,7 @@ const UNARY_ROUTES: UnaryRoutes = {
   'command.list': { schema: commandListRequestSchema, invoke: (api, r) => api.commands.list(r) },
   'command.execute': { schema: commandExecuteRequestSchema, invoke: (api, r, signal) => api.commands.execute(r, signal) },
   'skill.list': { schema: skillListRequestSchema, invoke: (api, r) => api.skills.list(r) },
-  'skill.invoke': { schema: skillInvokeRequestSchema, invoke: (api, r) => api.skills.invoke(r) },
+  'skill.invoke': { schema: skillInvokeRequestSchema, invoke: (api, r, signal) => api.skills.invoke(r, signal) },
   'goal.create': { schema: goalCreateRequestSchema, invoke: (api, r) => api.goals.create(r) },
   'goal.edit': { schema: goalEditRequestSchema, invoke: (api, r) => api.goals.edit(r) },
   'goal.pause': { schema: goalPauseRequestSchema, invoke: (api, r) => api.goals.pause(r) },