fix: guard POSIX-only process functions for Windows compatibility

os.setsid, os.killpg, and os.getpgid do not exist on Windows and raise
AttributeError on import or first call. This breaks the terminal tool,
code execution sandbox, process registry, and WhatsApp bridge on Windows.

Added _IS_WINDOWS platform guard in all four affected files, following
the pattern documented in CONTRIBUTING.md. On Windows, preexec_fn is
set to None and process termination falls back to proc.terminate() /
proc.kill() instead of process group signals.

Files changed:
- tools/environments/local.py (3 call sites)
- tools/process_registry.py (2 call sites)
- tools/code_execution_tool.py (3 call sites)
- gateway/platforms/whatsapp.py (3 call sites)

tools/process_registry.py

@@ -32,6 +32,7 @@ Usage:
 import json
 import logging
 import os
+import platform
 import shlex
 import shutil
 import signal
@@ -39,6 +40,8 @@ import subprocess
 import threading
 import time
 import uuid
+
+_IS_WINDOWS = platform.system() == "Windows"
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any, Dict, List, Optional
@@ -192,7 +195,7 @@ class ProcessRegistry:
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT,
             stdin=subprocess.PIPE,
-            preexec_fn=os.setsid,
+            preexec_fn=None if _IS_WINDOWS else os.setsid,
         )
 
         session.process = proc
@@ -544,7 +547,10 @@ class ProcessRegistry:
             elif session.process:
                 # Local process -- kill the process group
                 try:
-                    os.killpg(os.getpgid(session.process.pid), signal.SIGTERM)
+                    if _IS_WINDOWS:
+                        session.process.terminate()
+                    else:
+                        os.killpg(os.getpgid(session.process.pid), signal.SIGTERM)
                 except (ProcessLookupError, PermissionError):
                     session.process.kill()
             elif session.env_ref and session.pid: