fix: harden durable workspace runtime config leases

harness/tht/config.py

@@ -573,10 +573,34 @@ def _validate_raw_config_shape(raw: dict[str, Any], path: Path) -> None:
 
 
 def load_config(path: Path) -> Config:
-    if not path.exists():
-        raise ConfigError(f"File di configurazione non trovato: {path}")
+    # Backend runtime leases pass the verified canonical config as fd 3 while retaining
+    # the ordinary absolute -c argument for diagnostics and source identity.  Never reopen
+    # that pathname: an ancestor or leaf replacement after spawn must not alter bytes used
+    # by the harness.
+    runtime_fd = os.environ.get("THT_CONFIG_FD")
+    if runtime_fd is not None:
+        try:
+            fd = int(runtime_fd)
+            info = os.fstat(fd)
+            if (not stat.S_ISREG(info.st_mode) or info.st_nlink != 1
+                    or stat.S_IMODE(info.st_mode) != 0o400
+                    or info.st_uid != os.getuid()):
+                raise OSError("unsafe runtime config descriptor")
+            chunks: list[bytes] = []
+            while chunk := os.read(fd, 1024 * 1024):
+                chunks.append(chunk)
+            source_text = b"".join(chunks).decode("utf-8")
+        except (OSError, UnicodeError, ValueError) as exc:
+            raise ConfigError("File di configurazione runtime non attendibile") from exc
+    else:
+        if not path.exists():
+            raise ConfigError(f"File di configurazione non trovato: {path}")
+        try:
+            source_text = path.read_text()
+        except OSError as exc:
+            raise ConfigError(f"File di configurazione non trovato: {path}") from exc
     try:
-        raw = yaml.safe_load(path.read_text())
+        raw = yaml.safe_load(source_text)
     except yaml.YAMLError as exc:
         raise ConfigError(f"Configurazione YAML non valida: {path}") from exc
     if not isinstance(raw, dict):