When `gt formula run` fell back to the default "gastown" rig (because no
rig could be detected), it didn't set rigPath, which meant the default
formula lookup would fail. Now rigPath is properly constructed when we
have townRoot but can't detect a current rig.
Also adds tests for GetDefaultFormula helper.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>