Allow for prompt caching to work by using baibot_conversation_start_time_utc instead of baibot_now_utc

This patch introduces a new `baibot_conversation_start_time_utc`
variable which indicates the time the conversation got started.

Using `baibot_now_utc` is still possible, but given that the current
time is a moving target, its use is in conflict with prompt caching.

Because the new `baibot_conversation_start_time_utc` prompt variable
is a more reasonable default, we're now using it in all sample configs.

src/agent/provider/anthropic/controller.rs

@@ -72,6 +72,7 @@ impl ControllerTrait for Controller {
         let messages = vec![LLMMessage {
             author: LLMAuthor::User,
             message_text: "Hello!".to_string(),
+            timestamp: chrono::Utc::now(),
         }];
 
         let conversation = LLMConversation { messages };
@@ -108,6 +109,7 @@ impl ControllerTrait for Controller {
             Some(LLMMessage {
                 author: LLMAuthor::Prompt,
                 message_text: prompt_text,
+                timestamp: chrono::Utc::now(),
             })
         };