chore: install Playwright so the visual checks can actually run

`.agents/scripts/visual-regression.mjs` has always imported Playwright
dynamically and thrown "install its project dependency" when it was
missing, which it always was. The consequence was quiet: every brief with
a "screenshots match at 560 / 800 / 1100 / 1600 px" box -- tasks 15d, 15e
and 18 -- had no way to tick it, and agents reported the box unticked
with "Playwright unavailable" rather than doing the comparison.

The browsers were already cached in ~/.cache/ms-playwright; only the node
package was absent. Verified a headless screenshot works after install.

pnpm-lock.yaml changes here as a side effect of `pnpm add -D`, never by
hand. No package-lock.json, yarn.lock or bun.lock.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

package.json

@@ -24,6 +24,7 @@
     "eslint-plugin-astro": "^1",
     "husky": "^9",
     "lint-staged": "^16",
+    "playwright": "^1.62.1",
     "prettier": "^3",
     "prettier-plugin-astro": "^0.14",
     "stylelint": "^16",