fix(acp): keep per-session teardown failure reasons in the aggregate log

The connection-close teardown path threw a bare `AggregateError` whose
message counts the failed sessions, and its only production consumer logs
through `String(error)` — which renders the message alone. Compared with
the previous `Promise.all` behavior, every actual disposal failure reason
disappeared from operational logs.

Join the per-session reasons into the aggregate message, matching the
subagent seam's own aggregate disposal messages, and pin the reason in
the dispose spec's warning assertion.

packages/acp/acp/src/index.ts

@@ -368,7 +368,15 @@ export function apply(ctx: Context, config: AcpConfig): void {
         if (result.status === 'rejected') failures.push(result.reason as unknown)
       }
       if (failures.length > 0) {
-        throw new AggregateError(failures, `ACP agent teardown failed for ${failures.length} session(s)`)
+        // The only production consumer logs this error through `String`, which
+        // renders the message alone — without the joined reasons, per-session
+        // disposal failures would vanish from operational logs. Join them like
+        // the subagent seam's own aggregate disposal messages.
+        const detail = failures.map(failure => String(failure)).join('; ')
+        throw new AggregateError(
+          failures,
+          `ACP agent teardown failed for ${failures.length} session(s): ${detail}`,
+        )
       }
     })()
     return quiescing

packages/acp/acp/tests/dispose.spec.ts

@@ -131,7 +131,8 @@ describe('ACP connection ownership', () => {
 
     releaseSecond.resolve(undefined)
     await vi.waitFor(() => {
-      expect(warnings.some(warning => warning.includes('ACP agent teardown failed for 1 session(s)'))).toBe(true)
+      expect(warnings.some(warning =>
+        warning.includes('ACP agent teardown failed for 1 session(s): Error: first session cleanup failed'))).toBe(true)
       expect(harness!.ctx.agents.get(SessionId(first.sessionId))).toBeUndefined()
       expect(harness!.ctx.agents.get(SessionId(second.sessionId))).toBeUndefined()
     })