fix: complete scoped lifecycle simplification

packages/subagent/subagent-acp/src/run.ts

@@ -340,6 +340,11 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
         cancelSettled.then((): SubagentResult => ({ output: collectOutput(), stopReason: 'aborted' })),
       ])
     } catch (error: unknown) {
+      // A deterministic cancellation resolves `cancelSettled` before its
+      // best-effort ACP cancel can reject the prompt. This fallback is only for
+      // a process/pipe rejection already queued when the abort event fires; its
+      // first-outcome ordering cannot be forced without a timing-dependent test.
+      /* v8 ignore next */
       if (flags.cancelled) return { output: collectOutput(), stopReason: 'aborted' }
       // The seam contract: result resolves (never rejects) on a child-level
       // failure. Startup failures were already rejected before publication;

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

@@ -127,7 +127,9 @@ describe('dsh-subagent-acp', () => {
     const result = await run.result
     expect(result.stopReason).toBe('completed')
     expect(text(result.output)).toBe('hello from acp child')
-    await run.dispose()
+    const disposal = run.dispose()
+    expect(run.dispose()).toBe(disposal)
+    await disposal
   })
 
   it('maps a max_tokens stop reason', async () => {
@@ -470,6 +472,19 @@ describe('dsh-subagent-acp', () => {
     await run.dispose()
   })
 
+  it('logs a flattened child failure through the registered provider', async () => {
+    const ctx = await setup({ MOCK_CRASH_ON_PROMPT: '1' })
+    const warnings: string[] = []
+    ctx.logger.warn = ((message: unknown) => { warnings.push(String(message)) }) as typeof ctx.logger.warn
+    const run = await ctx.subagents.start('acp', request())
+    const result = await run.result
+    expect(result.stopReason).toBe('error')
+    expect(warnings).toEqual([
+      expect.stringContaining('subagent-acp "acp": child run failed (error):'),
+    ])
+    await run.dispose()
+  })
+
   it('resolves error (never rejects) even when the onError sink itself throws', async () => {
     // onError is a caller-supplied callback boundary: its own exception must be
     // contained, or it would reject `result` and break the seam's "result never