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/anthropic/config.rs

@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 
 use anthropic_rs::models::claude::ClaudeModel;
 
-use crate::agent::provider::ConfigTrait;
+use crate::agent::{default_prompt, provider::ConfigTrait};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Config {
@@ -58,7 +58,7 @@ 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: 8192,
             max_context_tokens: 204_800,