gitignore: exclude .claude/ (Claude Code agent runtime scratch)

Background agents launched via `isolation: "worktree"` register their
worktrees under `.claude/worktrees/` and leave session scratch around
the same directory. These are machine-local, reconstructable, and
should not travel with the repo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -9,5 +9,7 @@ composer.lock
 *.sqlite-journal
 /node_modules/
 /.phpunit.cache/
+# Claude Code agent runtime — worktrees + session scratch.
+/.claude/
 # Compiled by the Docker CSS builder stage (or `npm run build:css` for local dev).
 /public/assets/css/app.css