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/environments/local.py

@@ -1,12 +1,15 @@
 """Local execution environment with interrupt support and non-blocking I/O."""
 
 import os
+import platform
 import shutil
 import signal
 import subprocess
 import threading
 import time
 
+_IS_WINDOWS = platform.system() == "Windows"
+
 from tools.environments.base import BaseEnvironment
 
 # Noise lines emitted by interactive shells when stdin is not a terminal.
@@ -68,7 +71,7 @@ class LocalEnvironment(BaseEnvironment):
                 stdout=subprocess.PIPE,
                 stderr=subprocess.STDOUT,
                 stdin=subprocess.PIPE if stdin_data is not None else subprocess.DEVNULL,
-                preexec_fn=os.setsid,
+                preexec_fn=None if _IS_WINDOWS else os.setsid,
             )
 
             if stdin_data is not None:
@@ -101,12 +104,15 @@ class LocalEnvironment(BaseEnvironment):
             while proc.poll() is None:
                 if _interrupt_event.is_set():
                     try:
-                        pgid = os.getpgid(proc.pid)
-                        os.killpg(pgid, signal.SIGTERM)
-                        try:
-                            proc.wait(timeout=1.0)
-                        except subprocess.TimeoutExpired:
-                            os.killpg(pgid, signal.SIGKILL)
+                        if _IS_WINDOWS:
+                            proc.terminate()
+                        else:
+                            pgid = os.getpgid(proc.pid)
+                            os.killpg(pgid, signal.SIGTERM)
+                            try:
+                                proc.wait(timeout=1.0)
+                            except subprocess.TimeoutExpired:
+                                os.killpg(pgid, signal.SIGKILL)
                     except (ProcessLookupError, PermissionError):
                         proc.kill()
                     reader.join(timeout=2)
@@ -116,7 +122,10 @@ class LocalEnvironment(BaseEnvironment):
                     }
                 if time.monotonic() > deadline:
                     try:
-                        os.killpg(os.getpgid(proc.pid), signal.SIGTERM)
+                        if _IS_WINDOWS:
+                            proc.terminate()
+                        else:
+                            os.killpg(os.getpgid(proc.pid), signal.SIGTERM)
                     except (ProcessLookupError, PermissionError):
                         proc.kill()
                     reader.join(timeout=2)