Ignore generated Storybook/Argos screenshot output (#21589)

## Context

`packages/twenty-front/screenshots/` (and the equivalent under other
Storybook packages) holds Argos visual-regression baselines that are
generated on demand and not meant to be committed. The existing rules in
`.gitignore` were:

```
screenshots/
!**/screenshots/
```

The `!**/screenshots/` negation was added so app images committed under
`public/screenshots/` (github-connector) and `assets/screenshots/`
(codex-plugin) stay tracked — but it is too broad: it also re-includes
the generated output, so a `git add -A` can accidentally commit hundreds
of baseline PNGs.

## What this does

Narrows the negation to the two locations where screenshots are
intentionally committed:

```
screenshots/
!**/public/screenshots/
!**/assets/screenshots/
```

Generated `screenshots/` directories at package roots (twenty-front,
twenty-ui, twenty-ui-deprecated, twenty-front-component-renderer) are
now ignored again, while committed app images remain tracked. Verified
with `git check-ignore`.

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.gitignore

@@ -57,4 +57,5 @@ TRANSLATION_QA_REPORT.md
 .playwright-cli/
 output/playwright/
 screenshots/
-!**/screenshots/
+!**/public/screenshots/
+!**/assets/screenshots/