[Feature] Change the path of prompt.json saved by save to a relative path

When working on another project using the prompt.json backup file saved via /save, the working directory may change, potentially causing confusion in your work.

To support multiple concurrent projects, a project-specific state path is needed to store the prompt history up to the current point.

Start by tracing the /save command and the code that writes prompt.json, focusing on how the current working directory and project-specific state are determined. Done means concurrent projects keep separate prompt history without confusion when the working directory changes; the issue names no files or tests, so locate relevant coverage before making changes.

Written by the indexing model from the issue text.