refactor(agent): address inbox mutations by message id

packages/core/agent-loop/tests/agent.spec.ts

@@ -70,7 +70,7 @@ describe('Agent', () => {
       source: { kind: 'plugin', plugin: 'test' },
     })
     agent.inject(context)
-    agent.inbox.remove('next-step', context.id)
+    agent.inbox.remove(context.id)
     const prompt = createUserMessage({ content: [{ type: 'text', text: 'run' }], source: { kind: 'user' } })
     agent.followup(prompt)
     await agent.whenIdle()

packages/core/agent-loop/tests/contract-regressions.spec.ts

@@ -194,7 +194,7 @@ describe('abort during tool execution ends the turn', () => {
     send(agent, 'go')
     // The wake microtask has not run yet: remove the only pending message so
     // the admission batch is empty.
-    agent.inbox.remove('next-turn', agent.inbox.nextTurn[0]!.id)
+    agent.inbox.remove(agent.inbox.nextTurn[0]!.id)
     await waitForIdle(ctx, agent)
     expect(adapter.requests).toHaveLength(0)
     expect(agent.session.events.some(event => event.type === 'turn/start')).toBe(false)