feat: store and display backend + host metadata on chat messages

Each assistant message in the session JSON now carries:
  backend, backend_label, host (platform.node())

These fields are shown as model tags in the UI — on live responses and
when loading session history. Session log entries (sessions/YYYY-MM-DD.md)
include the backend label and host in the turn header.

The local (OpenAI-compat) backend strips non-standard fields before
sending messages to the API so extra fields don't leak upstream.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/session_logger.py

@@ -3,7 +3,13 @@ from config import settings
 from persona import persona_path
 
 
-def log_turn(session_id: str, user_msg: str, assistant_msg: str) -> None:
+def log_turn(
+    session_id: str,
+    user_msg: str,
+    assistant_msg: str,
+    backend_label: str = "",
+    host: str = "",
+) -> None:
     today = datetime.now().strftime("%Y-%m-%d")
     sessions_dir = persona_path() / "sessions"
     sessions_dir.mkdir(exist_ok=True)
@@ -12,11 +18,14 @@ def log_turn(session_id: str, user_msg: str, assistant_msg: str) -> None:
     timestamp = datetime.now().strftime("%H:%M")
     is_new = not log_file.exists()
 
+    meta_parts = [p for p in [backend_label, host] if p]
+    meta = f" · {' / '.join(meta_parts)}" if meta_parts else ""
+
     with open(log_file, "a") as f:
         if is_new:
             f.write(f"# Session Log — {today}\n")
         f.write(
-            f"\n### [{timestamp}] `{session_id}`\n"
+            f"\n### [{timestamp}] `{session_id}`{meta}\n"
             f"**{settings.user_name}:** {user_msg}\n\n"
             f"**{settings.agent_name}:** {assistant_msg}\n"
         )