refactor(subagent): unify async readiness and cancellation

packages/subagent/subagent-acp/tests/subagent-acp.e2e.ts

@@ -60,9 +60,10 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('ACP backend with-key e2e (drive
       },
     })
 
-    const run = ctx.subagents.start('acp', {
+    const run = await ctx.subagents.start('acp', {
       prompt: [{ type: 'text', text: 'Reply with exactly the word PONG and nothing else. Do not use any tools.' }],
       parent: fakeParent,
+      signal: new AbortController().signal,
     })
     const result = await run.result
     await run.dispose()
@@ -93,11 +94,12 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('ACP backend with-key e2e (drive
       },
     })
 
-    const run = ctx.subagents.start('acp', {
+    const run = await ctx.subagents.start('acp', {
       prompt: [{ type: 'text', text:
         'Use the bash tool to write the text ACP_CHILD_WAS_HERE into a file named proof.txt '
         + 'in the current directory. Then reply DONE.' }],
       parent: fakeParent,
+      signal: new AbortController().signal,
     })
     const result = await run.result
     await run.dispose()