Ensure Required Reference Check Emits

1. OpenSpec

• 1.1 Add governance requirement delta for required checks emitting terminal statuses.
• 1.2 Validate the OpenSpec change strictly.

2. Workflow

• 2.1 Remove the workflow-level pull-request path filter from .github/workflows/reference-implementation.yml.
• 2.2 Add merge_group support.
• 2.3 Add an in-job reference-impact classifier with the existing path set.
• 2.4 Gate expensive install/test steps on the classifier while keeping the required job name unchanged.

3. Validation

• 3.1 Run a local syntax/parse check for the workflow YAML.
• 3.2 Run openspec validate ensure-required-reference-check-emits --strict.
• 3.3 Run git diff --check.