Refactor: Externalize large prompt inside elisp code to dedicate .md file, under /prompt folder

[tninja]

Separate the large prompt and code, let code load the prompt when needed. When we need to use the prompt there, it should use @ to cite it, just like what we did for the harness .md file.

[tninja]

Right now these prompt are harness related. It got saved to "git-root"/.ai.code.files/harness. It introduce duplicates (and pollute?) into each individual project. Maybe a better place is the "folder-of-this-package"/prompt directory.

[tninja]

It is possible that the /prompt dir not got packaged properly and distributed. So when it is not there, the code should generate it firstly and cite it with @ .

[tninja]

After this got implemented, there is no need to keep the harness .md generate and cite logic. under "git-root"/.ai.code.files/harness

[tninja]

resolved by #376