fix(issue-45): restore stays off when python3 has no tomllib

The shim runs a bare python3. Before 3.11 the module failed to import and the
server started with restore on; now it gets the key alone, with a warning.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/lib_py/herdr_config.py

@@ -32,7 +32,6 @@ import os
 import re
 import sys
 import tempfile
-import tomllib
 from typing import List, Mapping, Optional, Tuple
 
 MARKER = "# generated by multi-agent-mux (Issue #45) - do not edit"
@@ -42,6 +41,11 @@ FALLBACK_NAME = "herdr-server-config.toml"
 KEY = "resume_agents_on_restore"
 MINIMAL = f"[session]\n{KEY} = false\n"
 
+try:
+    import tomllib
+except ImportError:  # Python < 3.11: no parser to check a merge with
+    tomllib = None
+
 _SESSION_HEADER = re.compile(r"^\s*\[\s*session\s*\]\s*(#.*)?$")
 _ANY_HEADER = re.compile(r"^\s*\[")
 _KEY_LINE = re.compile(rf"^\s*{KEY}\s*=")
@@ -107,6 +111,9 @@ def server_config_text(user_text: Optional[str]) -> Tuple[str, List[str]]:
     always has session.resume_agents_on_restore = false."""
     if not user_text or not user_text.strip():
         return MINIMAL, []
+    if tomllib is None:
+        return MINIMAL, [f"this Python has no tomllib (3.11+); the MAM server uses only {KEY} = false, "
+                         "without the rest of your settings"]
     try:
         user = tomllib.loads(user_text)
     except tomllib.TOMLDecodeError as e:

tests/test_issue45_herdr_config.py

@@ -71,6 +71,14 @@ def test_a_bad_value_type_falls_back_to_minimal():
     assert warnings == []
 
 
+def test_without_tomllib_restore_is_still_off(monkeypatch):
+    # The shim runs a bare `python3`; on one older than 3.11 an import error
+    # would leave the server with restore on. Off wins over the user's settings.
+    monkeypatch.setattr(hc, "tomllib", None)
+    text, warnings = hc.server_config_text("[ui]\nconfirm_close = false\n")
+    assert text == hc.MINIMAL and "no tomllib" in warnings[0]
+
+
 def _env(tmp_path, **kw):
     env = {"HOME": str(tmp_path / "home")}
     env.update(kw)