fix: test local working tree in quickstart test (#938)

The quickstart test installed the theme via `git submodule add` from the
published remote, so CI (which checks out the PR) and the pre-push hook
never exercised the local code under test — a broken change could pass.

Install the local working tree as the theme by default (via `git ls-files`,
including uncommitted/untracked changes while skipping ignored paths), and
keep the documented published-submodule flow behind `--use-submodule`.

- scripts: add `--theme-path` / `--use-submodule`, default to local mode
- ci: run local mode on every push/PR, submodule mode post-merge on main
- package.json: add `test:quickstart:submodule`

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>