refactor(host)!: retire the skill.invoke RPC for the gesture boundary

Invocation is an ordinary session.prompt again: the pre-step gesture
boundary makes it deterministic host-side for every front end, so the
dedicated RPC (handler, wire schema, error codes, client face, fixtures)
and ui-skill's claim machinery are net deletions. The menu keeps decision
21 exactly — a pick lands literal /name text — plus the user-only marker
from skill.list's modelInvocable flag.

packages/host/apiproxy/src/api/skills.schema.ts

@@ -26,18 +26,3 @@ export const skillListRequestSchema = z.object({
 export const skillListValueSchema = z.object({
   skills: z.array(skillEntrySchema),
 }) satisfies z.ZodType<Wire<ResponseValue<'skill.list'>>>
-
-/**
- * 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().min(1).optional(),
-}) satisfies z.ZodType<Wire<RequestPayload<'skill.invoke'>>>
-
-/** skill.invoke response value. */
-export const skillInvokeValueSchema = z.object({
-  accepted: z.literal(true),
-}) satisfies z.ZodType<Wire<ResponseValue<'skill.invoke'>>>