refactor(agent): unify sourced message delivery

examples/headless-agent/tests/code-mode.e2e.ts

@@ -311,12 +311,12 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('Code Mode: real model writes a p
     ctx = await codeModeHarness(workdir)
     const agent = ctx.agentLoop.create(SessionId('e2e-code-mode'), { provider: 'deepseek', model: 'deepseek-v4-flash' })
 
-    agent.followup([{
+    agent.followup({ content: [{
       type: 'text',
       text: 'Using one run_code program: run `echo alpha-7` with the bash tool, run `echo beta-9` with the bash tool, '
         + 'then write both outputs joined by a plus sign into combined.txt (bash heredoc or redirect), '
         + 'and return only the joined string.',
-    }])
+    }], source: { kind: 'user' } })
     await waitForIdle(ctx, agent)
     const events: SessionEvent[] = [...agent.session.events]
 
@@ -363,10 +363,10 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('Code Mode: real model writes a p
       agentOptions: { provider: 'deepseek', model: 'deepseek-v4-flash' },
     })
 
-    handle.agent.followup([{
+    handle.agent.followup({ content: [{
       type: 'text',
       text: 'Use one run_code program to call tools.read on pkg/deep/task.txt. After it finishes, answer: Code Mode workspace handshake?',
-    }])
+    }], source: { kind: 'user' } })
     await waitForIdle(ctx, handle.agent)
 
     const events: SessionEvent[] = [...handle.agent.session.events]