fix(shell): reap zombie processes on all subprocess exit paths

The previous fix (dbcc7cb5) only added os.waitpid() to _kill_process(),
covering the timeout/cancel path of one-shot exec. Zombies continued to
accumulate because several other exit paths never reaped children:

- _ExecSession.kill(): sent SIGKILL + process.wait(5s) but had no
  os.waitpid() fallback if the wait timed out
- ExecTool.execute() generic exception handler: leaked the subprocess
  if communicate() raised an unexpected error
- Normal completion paths: relied entirely on asyncio's child-watcher,
  which can miss exits inside Docker containers (pidfd/SIGCHLD gaps)

Changes:
- Extract _reap_pid() helper for consistent, safe os.waitpid(WNOHANG)
- Add _reap_pid() fallback to _ExecSession.kill() via try/finally
- Add _reap_pid() safety-net after normal process exit in both
  ExecTool.execute() and _ExecSession.poll()
- Kill + reap subprocess in the generic except Exception handler
- Add periodic zombie reaper background task (every 30s) in the
  gateway as a last line of defense

nanobot/cli/commands.py

@@ -201,6 +201,7 @@ app = typer.Typer(
 )
 
 console = Console()
+_IS_WINDOWS = sys.platform == "win32"
 EXIT_COMMANDS = {"exit", "quit", "/exit", "/quit", ":q"}
 _REASONING_SENTENCE_ENDINGS = (".", "!", "?", "。", "！", "？")
 _REASONING_FLUSH_CHARS = 60
@@ -1695,6 +1696,29 @@ def _run_gateway(
     else:
         console.print("[yellow]✗[/yellow] Heartbeat: disabled")
 
+    async def _zombie_reaper() -> None:
+        """Periodically reap zombie child processes.
+
+        asyncio's child-watcher *should* reap all children, but inside
+        Docker containers the pidfd / SIGCHLD mechanism can miss exits.
+        This task runs every 30 s and calls ``os.waitpid(-1, WNOHANG)``
+        in a loop to collect any zombies that slipped through.
+        """
+        _INTERVAL = 30
+        while True:
+            await asyncio.sleep(_INTERVAL)
+            reaped = 0
+            while True:
+                try:
+                    pid, _ = os.waitpid(-1, os.WNOHANG)
+                    if pid == 0:
+                        break  # no more zombie children
+                    reaped += 1
+                except ChildProcessError:
+                    break  # no child processes at all
+            if reaped:
+                logger.info("Zombie reaper: reaped {} defunct child process(es)", reaped)
+
     async def _health_server(host: str, health_port: int):
         """Lightweight HTTP health endpoint on the gateway port."""
         import json as _json
@@ -1820,6 +1844,11 @@ def _run_gateway(
                     name="nanobot-local-triggers",
                 ),
             ]
+            if not _IS_WINDOWS:
+                tasks.append(asyncio.create_task(
+                    _zombie_reaper(),
+                    name="nanobot-zombie-reaper",
+                ))
             if health_server_enabled:
                 tasks.append(asyncio.create_task(
                     _health_server(config.gateway.host, port),