build: keep agent worktrees out of prettier and git

Running implementation agents with worktree isolation puts full checkouts of
this repo under .claude/worktrees/. `prettier --check .` walks into them, so
`npm run ci` went red on 70 files that belong to another checkout — including
the vendored CIBG design system, which the top-level ignore already excludes.

Ignored in both .prettierignore and .gitignore; the latter so a worktree can
never be committed into the repo it is a checkout of.

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

.gitignore

@@ -58,3 +58,7 @@ backend/openzaak/seeded.env
 
 # WP-55: render-prod-secrets.sh's output — the real client secret, never committed
 backend/openzaak/setup_configuration/data.prod.yaml
+
+# Agent git worktrees (Claude Code `isolation: "worktree"`) — full checkouts of
+# this repo nested inside it; never commit one.
+.claude/worktrees/

.prettierignore

@@ -4,6 +4,11 @@ storybook-static*/
 coverage/
 .angular/
 
+# Agent git worktrees — full checkouts of this repo nested inside it, so an
+# unignored `prettier --check .` walks into every one of them (and reports the
+# vendored CIBG files that the top-level ignore already excludes).
+.claude/worktrees/
+
 # Lockfile
 package-lock.json