The new `--new` or `-n` flag allows to create a new revision after the
single revision being described. Running `jj-cz --new` is the
equivalent of running `jj-cz @ && jj new`. Running `jj-cz --new xs` is
the equivalent of running `jj-cz xs && jj new xs`.
Passing several revisions to `jj-cz` with the `--new` flag will result
in an error.
Refs: #6
Replace CLI executor with jj-lib integration, implement full interactive
commit workflow via prompts, and add mock infrastructure for testing.
Add CLI integration tests and error handling tests.
phundrak
