feature: caller-customizable help/instruction text

[arxanas]

Related to/inspired by jj-vcs/jj#4984. The "dummy file with instructions" UI is suboptimal and primarily a concession to having less-well-integrated diff editors. It would be better if there were a natively-supported way to embed the instructions in the UI.

[aside] The instructions for scm-record will be different than most other editors. The JJ-INSTRUCTIONS file there makes reference to "left" and "right" sides, which don't currently exist in scm-record.

It would also be helpful to include the version and/or project URLs of the caller as well as of scm-record itself, which would help with bug reporting. It might go in the same UI component or a different one.