Generate pre-commit hook setup for `agebox validate --no-decrypt`

[evangineer]

Optional pre-commit hook generation at init time perhaps?

agebox init --precommit-validate

[evangineer]

In the meantime, I'm implementing this: https://github.com/gitleaks/gitleaks

[evangineer]

I did implement a gitleaks pre-commit hook and it makes an excellent combo with agebox.

[evangineer]

This overlaps with #146