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/api/skills.schema.ts

@@ -27,11 +27,14 @@ export const skillListValueSchema = z.object({
   skills: z.array(skillEntrySchema),
 }) satisfies z.ZodType<Wire<ResponseValue<'skill.list'>>>
 
-/** skill.invoke request payload. */
+/**
+ * skill.invoke request payload. `text` is the user's trailing message; a
+ * blank one stays off the wire (the boundary, not client courtesy, refuses it).
+ */
 export const skillInvokeRequestSchema = z.object({
   sessionId: sessionIdSchema,
   name: z.string().min(1),
-  text: z.string().optional(),
+  text: z.string().min(1).optional(),
 }) satisfies z.ZodType<Wire<RequestPayload<'skill.invoke'>>>
 
 /** skill.invoke response value. */