fix(subagent): separate message prefix from body

packages/subagent/subagent/src/continuation.ts

@@ -294,7 +294,7 @@ function agentMessageSource(sender: Agent): AgentMessageSource {
 function agentMessage(sender: Agent, content: ContentBlock[]) {
   return createUserMessage({
     content: [
-      { type: 'text' as const, text: `Agent ${sender.id} sent a message:` },
+      { type: 'text' as const, text: `Agent ${sender.id} sent a message: ` },
       ...content,
     ],
     source: agentMessageSource(sender),

packages/subagent/subagent/tests/continuation.spec.ts

@@ -1954,7 +1954,7 @@ describe('continuable adjacent-Agent delivery', () => {
       senderSessionId: started.childId,
     })
     expect(delivered?.content).toEqual([
-      { type: 'text', text: `Agent ${started.childId} sent a message:` },
+      { type: 'text', text: `Agent ${started.childId} sent a message: ` },
       { type: 'text', text: 'an explicit message' },
     ])
 

packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts

@@ -221,7 +221,7 @@ describe('dsh-tool-subagent-control', () => {
       senderSessionId: started.childId,
     })
     expect(delivered[0]?.message.content).toEqual([
-      { type: 'text', text: `Agent ${started.childId} sent a message:` },
+      { type: 'text', text: `Agent ${started.childId} sent a message: ` },
       { type: 'text', text: 'CHILD_FINDING' },
     ])
 
@@ -257,7 +257,7 @@ describe('dsh-tool-subagent-control', () => {
       senderSessionId: parent.id,
     })
     expect(followUp?.type === 'user/message' && followUp.data.content).toEqual([
-      { type: 'text', text: `Agent ${parent.id} sent a message:` },
+      { type: 'text', text: `Agent ${parent.id} sent a message: ` },
       { type: 'text', text: 'and then?' },
     ])
   })
@@ -288,7 +288,7 @@ describe('dsh-tool-subagent-control', () => {
       : [])
     expect(prompts).toEqual([
       'long work',
-      `Agent ${parent.id} sent a message:`,
+      `Agent ${parent.id} sent a message: `,
       'also consider Y',
     ])
   })
@@ -411,9 +411,9 @@ describe('dsh-tool-subagent-control interrupt_agent', () => {
       : [])
     expect(prompts).toEqual([
       'long work',
-      `Agent ${parent.id} sent a message:`,
+      `Agent ${parent.id} sent a message: `,
       'parked follow-up',
-      `Agent ${parent.id} sent a message:`,
+      `Agent ${parent.id} sent a message: `,
       'wake up',
     ])
   })