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/client/connection/tests/fake-api.ts

@@ -163,8 +163,6 @@ export class FakeApiClient implements IApiClient {
   onSkillList: (payload: unknown) => Promise<RpcResponse<{ skills: SkillEntry[] }>>
     = () => Promise.resolve(ok({ skills: [] }))
 
-  onSkillInvoke: (payload: unknown) => Promise<RpcResponse<{ accepted: true }>>
-    = () => Promise.resolve(ok({ accepted: true as const }))
 
   readonly commands: IApiClient['commands'] = {
     list: (payload: unknown) => this.record('command.list', payload, this.onCommandList(payload)),
@@ -173,7 +171,6 @@ export class FakeApiClient implements IApiClient {
 
   readonly skills: IApiClient['skills'] = {
     list: (payload: unknown) => this.record('skill.list', payload, this.onSkillList(payload)),
-    invoke: (payload: unknown) => this.record('skill.invoke', payload, this.onSkillInvoke(payload)),
   }
 
   readonly goals: IApiClient['goals'] = {