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.

src/agent/provider/openai_compat/config.rs

@@ -1,5 +1,6 @@
 use serde::{Deserialize, Serialize};
 
+use crate::agent::default_prompt;
 use crate::agent::provider::openai::{
     ImageGenerationConfig as OpenAIImageGenerationConfig,
     SpeechToTextConfig as OpenAISpeechToTextConfig,
@@ -75,7 +76,7 @@ impl Default for TextGenerationConfig {
     fn default() -> Self {
         Self {
             model_id: default_text_model_id(),
-            prompt: Some("You are a brief, but helpful bot.".to_owned()),
+            prompt: Some(default_prompt().to_owned()),
             temperature: super::super::default_temperature(),
             max_response_tokens: 4096,
             max_context_tokens: 128_000,