v0.1.7 Refined add and amend flag usage

✨ Features
- Refine `add` and `amend` command flag usage:
  - `-a` flag now acts as shorthand for `--add` INSTEAD OF AMEND to more
    closely follow `git commit` syntax
  - `--amend` is now exclusively a long-form flag (short flag
    removed).
  - Documentation updated across `main.go`, `AGENTS.md`, and
    `README.md` to reflect these changes.
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