fix(cli): synchronize HERMES_SESSION_ID across environment and contextvar during session switches

gateway/session_context.py

@@ -83,6 +83,21 @@ _VAR_MAP = {
 }
 
 
+def set_current_session_id(session_id: str) -> None:
+    """Synchronize ``HERMES_SESSION_ID`` across ContextVar and ``os.environ``.
+
+    Long-lived single-process entrypoints like the CLI can rotate sessions via
+    ``/new``, ``/resume``, ``/branch``, or compression splits without
+    reconstructing the entire agent. Tools still consult
+    ``get_session_env("HERMES_SESSION_ID")`` with an ``os.environ`` fallback,
+    so both storage paths must move together when the active session changes.
+    """
+    import os
+
+    os.environ["HERMES_SESSION_ID"] = session_id
+    _SESSION_ID.set(session_id)
+
+
 def set_session_vars(
     platform: str = "",
     chat_id: str = "",