mb init: should add jcard.toml to .gitignore

## Context

Related to #24 — `mb init` scaffolds a `jcard.toml` in the user's repo. Since developers may run `mb init` inside the masterblaster repo during development/testing, `jcard.toml` should be listed in `.gitignore` to avoid accidentally committing it.

## Solution

Add `jcard.toml` to the project's `.gitignore`.