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/llm_client.py

@@ -183,7 +183,8 @@ async def _local(system_prompt: str, messages: list[dict], model_cfg: dict | Non
     msgs: list[dict] = []
     if system_prompt:
         msgs.append({"role": "system", "content": system_prompt})
-    msgs.extend(messages)
+    # Strip any non-standard metadata fields before sending to the API
+    msgs.extend({"role": m["role"], "content": m["content"]} for m in messages)
 
     url = api_url.rstrip("/") + chat_path
     headers: dict[str, str] = {}