fix(kanban): serialize DB initialization

tests/hermes_cli/test_kanban_db_init.py

@@ -0,0 +1,38 @@
+from __future__ import annotations
+
+import threading
+from pathlib import Path
+
+from hermes_cli import kanban_db as kb
+
+
+def test_connect_initialization_is_thread_safe(tmp_path, monkeypatch):
+    home = tmp_path / ".hermes"
+    home.mkdir()
+    monkeypatch.setenv("HERMES_HOME", str(home))
+    monkeypatch.setattr(Path, "home", lambda: tmp_path)
+
+    db_path = kb.kanban_db_path(board="default")
+    kb._INITIALIZED_PATHS.discard(str(db_path.resolve()))
+
+    errors: list[BaseException] = []
+    barrier = threading.Barrier(8)
+
+    def worker() -> None:
+        try:
+            barrier.wait(timeout=5)
+            conn = kb.connect(board="default")
+            conn.close()
+        except BaseException as exc:  # pragma: no cover - surfaced below
+            errors.append(exc)
+
+    threads = [threading.Thread(target=worker) for _ in range(8)]
+    for thread in threads:
+        thread.start()
+    for thread in threads:
+        thread.join(timeout=10)
+
+    assert errors == []
+    with kb.connect(board="default") as conn:
+        cols = {row["name"] for row in conn.execute("PRAGMA table_info(tasks)")}
+    assert "max_retries" in cols