Make the buildingsync-measures-gem measures loadable #19

Identified during #13

Making buildingsync-measures-gem measures loadable.

The simplest route is the external measure manifest that BOSS already has. Most of the plumbing exists:

• The baseline writer already appends installed external measure folders to every OSW's measure paths.
• CI already runs the install task before tests.
• The gem's measures need nothing outside their own folders, so cloning only the measure folder works.

What's missing is one manifest entry and an install step:

1. Add the repo to external_measure_repos.yml:

  - name: buildingsync-measures-gem
    url: https://github.com/BuildingSync/buildingsync-measures-gem.git
    ref: <commit SHA on develop>
    measure_roots:
      - lib/measures

Pin a commit SHA rather than develop. The plan's rule is to map only to merged measures, and a moving branch can change measure arguments without warning.

2. Install locally with bundle exec rake measures:install_external. Nothing is installed under the vendor folder on your machine yet.

3. Add a test that every mapped measure name resolves from the written OSW's measure paths. PR 12 already lists that as its acceptance gate.

4. Watch for name clashes. Gem measures take precedence over external repos, and ComStock is also in the manifest. If ComStock has a measure with the same folder name, the earlier one wins. I couldn't check this because ComStock isn't installed here.

Installing it as a Ruby gem is the other route, but the gem is not published on RubyGems: the lookup returned 404, so the gem's own README install instructions don't work yet.