remove file listing from system prompt

[heimoshuiyu]

As described in #5224, changes in the list of files can cause cache invalidation, increasing API call costs and latency.

After removing the files list, I tested multiple models—GLM4.6, Gork 4 Fast, Gemini 3 Pro Preview, Qwen 3 Coder Plus, and Claude Sonnet 4.5—using 100M tokens in practical work tasks, and they all performed well.

I believe we can now remove the file list. The point raised in #2108 (comment) is valid—it’s something that changes dynamically. I think if we need to include project information in the coding agent, using Agents.md is more appropriate. Adding a command /init to help users initialize an Agents.md for a new project, where the directory structure and the locations of important files are described, would be a better approach. And that will be another PR...