refactor(event): remove state events

[tonyfettes]

Motivation

State events like ModelLoaded, ToolAdded, and SystemPromptSet were used in cmd/test and cmd/jsonl2md to show system information in logs. When implementing event storage and resuming functionality (#787), these event will be added to history for every resumption.

For example, if we created a new conversation, before this PR, the conversation would have the following events if #787 were merged:

• ModelLoaded
• ToolUpdated: read_file
• ToolUpdated: meta_write_file
• SystemPromptSet: You are a helpful assistant
• PreConversation
• UserMessage: Hello
• RequestCompleted: Hi! ...
• PostConversation

Then, if we were resuming such conversation using the Maria::resume implemented in #787, the history will have the following event:

• (old) ModelLoaded
• (old) ToolUpdated: read_file
• (old) ToolUpdated: meta_write_file
• (old) SystemPromptSet: You are a helpful assistant
• (old) PreConversation
• (old) UserMessage: Hello
• (old) RequestCompleted: Hi! ...
• (old) PostConversation
• (new) ModelLoaded
• (new) ToolUpdated: read_file
• (new) ToolUpdated: meta_write_file
• (new) SystemPromptSet: You are a helpful assistant

We found out that ModelLoaded, ToolUpdated, and SystemPromptSet events are duplicated.

Changes

This PR remove the ModelLoaded and ToolUpdated event, so that these events does not duplicated when resuming a conversation.

In order for cmd/test and cmd/jsonl2md to continue to work, we moved these events into cmd/test package, by defining type @cmd/test.Event and @cmd/test.EventDesc