chore: ignore tmp/, screenshots/, test-screenshots/ scratch dirs (#13042)

## Problem

Several recent PRs accidentally committed agent-generated screenshots
and tmp artifacts. These dirs are agent-browser / pr-test scratch space
— they should never end up in the repo.

Recent accidents:
- `34ca2e6170` (reverted in PR #13023) —
`tmp/pr-13023-screenshots/*.png`
- `f18eb09837` (PR #13031) — `test-screenshots/PR-13031/*.png`
- `6d78fbd09c` (PR #13027) — `screenshots/01-login-page.png`, etc.
- `a45eea49af` — `screenshots/test*-*.png`

## Changes

Add a new section near the bottom of `.gitignore`:

```
# Agent-generated scratch dirs (pr-test screenshots, agent-browser artifacts).
# See PR #13023 retro: these were committed by mistake several times.
tmp/
screenshots/
test-screenshots/
```

## No source files affected

`git ls-files | grep -E '^(tmp|screenshots|test-screenshots)/'` returns
no matches on `dev`. Nothing legitimate is being ignored — the broad
patterns are safe.

This is a defensive change; no runtime behavior impact.

## Test plan

- [x] Confirmed `git status` ignores synthetic files in those paths:
  ```
  $ mkdir -p tmp screenshots test-screenshots
  $ touch tmp/test.png screenshots/test.png test-screenshots/test.png
  $ git status --short
   M .gitignore
$ git check-ignore -v tmp/test.png screenshots/test.png
test-screenshots/test.png
  .gitignore:206:tmp/	tmp/test.png
  .gitignore:207:screenshots/	screenshots/test.png
  .gitignore:208:test-screenshots/	test-screenshots/test.png
  ```