fix: keep ACP stdout protocol-clean

Route AIAgent print output to stderr via _print_fn for ACP stdio sessions.
Gate quiet-mode spinner startup on _should_start_quiet_spinner() so JSON-RPC
on stdout isn't corrupted. Child agents inherit the redirect.

Co-authored-by: Git-on-my-level <Git-on-my-level@users.noreply.github.com>

acp_adapter/session.py

@@ -13,6 +13,7 @@ from hermes_constants import get_hermes_home
 import copy
 import json
 import logging
+import sys
 import uuid
 from dataclasses import dataclass, field
 from threading import Lock
@@ -21,6 +22,17 @@ from typing import Any, Dict, List, Optional
 logger = logging.getLogger(__name__)
 
 
+def _acp_stderr_print(*args, **kwargs) -> None:
+    """Best-effort human-readable output sink for ACP stdio sessions.
+
+    ACP reserves stdout for JSON-RPC frames, so any incidental CLI/status output
+    from AIAgent must be redirected away from stdout. Route it to stderr instead.
+    """
+    kwargs = dict(kwargs)
+    kwargs.setdefault("file", sys.stderr)
+    print(*args, **kwargs)
+
+
 def _register_task_cwd(task_id: str, cwd: str) -> None:
     """Bind a task/session id to the editor's working directory for tools."""
     if not task_id:
@@ -458,4 +470,8 @@ class SessionManager:
             logger.debug("ACP session falling back to default provider resolution", exc_info=True)
 
         _register_task_cwd(session_id, cwd)
-        return AIAgent(**kwargs)
+        agent = AIAgent(**kwargs)
+        # ACP stdio transport requires stdout to remain protocol-only JSON-RPC.
+        # Route any incidental human-readable agent output to stderr instead.
+        agent._print_fn = _acp_stderr_print
+        return agent