refactor(agent): disable system prompt setting

[tonyfettes]

Motivation

As mentioned in PR #839, events on fixed system attribute will be duplicated, if #787 is introduced. Specifically, this means if we have a event history below:

• ModelLoaded
• ToolAdded: read_file
• ToolAdded: write_to_file
• SystemPromptSet: you are a helpful assistant.
• PreConversation
• UserMessage: hello
• RequestCompleted: Hi, ...
• PostConversation

Such history will have duplicated events when resuming from disk:

• (old) ModelLoaded
• (old) ToolAdded: read_file
• (old) ToolAdded: write_to_file
• (old) SystemPromptSet: you are a helpful assistant.
• (old) PreConversation
• (old) UserMessage: hello
• (old) RequestCompleted: Hi, ...
• (old) PostConversation
• (new) ModelLoaded
• (new) ToolAdded: read_file
• (new) ToolAdded: write_to_file
• (new) SystemPromptSet: you are a helpful assistant.

Changes

This PR:

1. Restructured the code so that the system prompt is fixed after the agent is created, and not changed afterward. This holds true even in refactor(conversation): support resuming all events #787, meaning that even if a conversation is resumed, the system prompt is still the original one that the agent is created with, and cannot be changed.

2. Removed the existing API endpoints to set system-prompt:

   • Server: POST /v1/system-prompt
   • Daemon: POST /v1/task/{id}/system-prompt

   These api have not yet been used in ui, but they are used by the debug web interface. However, the GET /v1/system-prompt is kept for informational functionality and testing.

3. Does not removed the SystemPromptSet event. The removal of SystemPromptSet fit more into refactor(event): remove state events #839, not here.

[tonyfettes]

Closed, because we think it is ok for user to change the system prompt during the conversation. However, we think we should get rid of the SystemPromptSet event because storing multiple SystemPromptSet is expensive, because system prompt are very likely to be pretty long.