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/config.py

@@ -157,6 +157,17 @@ class Config:
     def token(self, terminal):
         return os.environ.get(self.token_env(terminal))
 
+    def secret_keys(self):
+        """Every env var name that carries a secret (Plane tokens, webhook secrets): never handed to executors."""
+        keys = {t["token_key"] for t in self.terminals.values()}
+        keys |= {p["webhook_secret_key"] for p in self.projects.values() if p.get("webhook_secret_key")}
+        return keys
+
+    def scrubbed_env(self):
+        """os.environ without the secrets above, for every child process agentd starts."""
+        drop = self.secret_keys()
+        return {k: v for k, v in os.environ.items() if k not in drop}
+
     def secret_env_args(self):
         """`--secret-env` arguments for agent-access.py exec, one per distinct token key."""
         keys = sorted({t["token_key"] for t in self.terminals.values()})