refactor(agent-loop): simplify message machine

packages/todo/tool-todo/tests/invariant.spec.ts

@@ -47,7 +47,7 @@ describe('todo snapshot invariants', () => {
     expect(() => {
       ctx.emit('tools/change')
       ctx.emit('session/event', {} as Session, {
-        type: 'turn/start', seq: 0, time: 0, data: { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } },
+        type: 'turn/start', seq: 0, time: 0, data: { turn: 1 },
       })
     }).not.toThrow()
   })

packages/todo/tool-todo/tests/projection.spec.ts

@@ -99,7 +99,7 @@ describe('todos projection provider', () => {
     session.append('todo/write', { todos: list })
     session.append('turn/end', { turn: 0, reason: { kind: 'completed' } })
     expect((await bench.tailProjections())?.values.todos).toEqual(list)
-    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    session.append('turn/start', { turn: 1 })
     const cleared = await bench.tailProjections()
     expect(cleared?.values.todos).toBeNull()
     expect(cleared?.asOfSeq).toBe(session.seq - 1)