feat(ui-skill): claim slash skill references into skill.invoke

A menu pick or an entered /name line now claims the composer into an
args-tolerant skill.invoke transaction instead of shipping the literal
text and hoping the model loads the skill. This gives every user-invocable
skill a deterministic entry point — including disable-model-invocation
skills the catalog never shows the model (issue #1470). Candidates carry
a user-only hint, and the unreached legacy <skill> reference codec is
removed (decision 21 removal cut).

packages/client/runtime/tests/fake-api.ts

@@ -198,6 +198,9 @@ 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)),
     execute: (payload: unknown) => this.record('command.execute', payload, this.onCommandExecute(payload)),
@@ -205,6 +208,7 @@ 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'] = {