Add support for prompt variables (bot name, date/time, model id)

Fixes https://github.com/etkecc/baibot/issues/10

This also includes them in the default prompts (for newly-created agents),
so that people can get a better experience out of the box.

docs/sample-provider-configs/ollama.yml

@@ -2,7 +2,7 @@ base_url: http://my-ollama-self-hosted-service:11434/v1
 api_key: YOUR_API_KEY_HERE
 text_generation:
   model_id: gemma2:2b
-  prompt: You are a brief, but helpful bot.
+  prompt: "You are a brief, but helpful bot called {{ baibot_name }} powered by the {{ baibot_model_id }} model. The date/time now is: {{ baibot_now_utc }}."
   temperature: 1.0
   max_response_tokens: 4096
   max_context_tokens: 128000