Harden transports and state files

- Executors and relay helpers get an environment without any configured
  Plane token / webhook secret (design 8: executors hold no Plane credentials).
- Windows runner: Set-Location -LiteralPath, and a failed cd writes exit=97
  instead of running the agent in the wrong directory.
- schtasks launch: escape apostrophes and pass the script path as \"...\" so
  paths with spaces survive PowerShell 5.1 native quoting.
- State and heartbeat files are written temp + fsync + rename.
- Tests for the above plus codex run/resume command shapes and unique run dirs.

agentd/transports/local.py

@@ -59,7 +59,9 @@ class LocalTransport:
     def launch(self, task, script_path, run_dir):
         # `setsid -f` double-forks: the runner leads its own session (pgid == pid, see kill) and is reparented
         # to init, so a long-running `tick --loop` never accumulates zombies.
+        # The executor must not hold Plane credentials (design §8): drop every configured secret from its env.
         subprocess.run(["setsid", "-f", "bash", script_path], cwd=run_dir, check=True, close_fds=True,
+                       env=self.cfg.scrubbed_env(),
                        stdin=subprocess.DEVNULL, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
 
     def kill(self, task, run_dir):