Add a gitignore feature module to `add`

## Summary
Add `stacksmith add gitignore` to write a sensible Node `.gitignore` into an existing project.

## Tasks
- [ ] New `FeatureModule` (id: `gitignore`) in `src/core/features/index.ts`
- [ ] Skip gracefully if `.gitignore` already exists (the runner already handles this)
- [ ] Cover it in `features.test.ts`

## Files
- `src/core/features/index.ts`

Start in `src/core/features/index.ts` and inspect how existing `FeatureModule`s are registered so you can add a `gitignore` module in the same pattern. Then open `features.test.ts` and add/adjust tests for `add gitignore` plus the skip behavior when `.gitignore` already exists. Run the feature tests for this area and finish when the module is registered and the new coverage passes.

Written by the indexing model from the issue text.