feat: inject model and provider into system prompt

Adds model name and provider to the system prompt metadata block,
alongside the existing session ID and timestamp. These are frozen
at session start and don't change mid-conversation, so they won't
break prompt caching.

run_agent.py

@@ -2061,6 +2061,10 @@ class AIAgent:
         timestamp_line = f"Conversation started: {now.strftime('%A, %B %d, %Y %I:%M %p')}"
         if self.pass_session_id and self.session_id:
             timestamp_line += f"\nSession ID: {self.session_id}"
+        if self.model:
+            timestamp_line += f"\nModel: {self.model}"
+        if self.provider:
+            timestamp_line += f"\nProvider: {self.provider}"
         prompt_parts.append(timestamp_line)
 
         platform_key = (self.platform or "").lower().strip()