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/mod.rs

@@ -19,3 +19,7 @@ pub use instantiation::Result as AgentInstantiationResult;
 
 pub use provider::{AgentProvider, AgentProviderInfo, ControllerTrait};
 pub use purpose::AgentPurpose;
+
+pub(super) fn default_prompt() -> &'static str {
+    "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 }}."
+}