Adds RequiredWorkflowsCheck to the drift checker. The check reads
required_workflows lists from drift-checker.config.json per repo type
(cursor-plugin, mcp-server) and emits an error finding for each absent
workflow. Policy is additive across config tiers (globals -> type ->
repo); extra workflows are never flagged; skip_checks suppresses the
check entirely.

New files:
- scripts/drift_check/checks/required_workflows.py
- tests/test_required_workflows.py (9 tests, all pass)

Changed files:
- types.py: adds required_workflows to RepoConfig, present_workflows to
  RepoSnapshot, and tier-union logic in DriftConfig.resolve()
- snapshot.py: discovers .github/workflows/ filenames into present_workflows
- cli.py: wires RequiredWorkflowsCheck into the check pipeline
- checks/__init__.py: exports RequiredWorkflowsCheck
- standards/drift-checker.config.json: adds required_workflows arrays for
  cursor-plugin and mcp-server types

On first run the check correctly flags two real gaps:
- steam-mcp: missing stale.yml (known from the prior manual audit)
- Mobile-App-Developer-Tools: missing stale.yml (caught by the check;
  missed by the manual audit)

Both are follow-up fix: PRs in their respective repos. They are not
suppressed here; the check is working as intended.

file=None findings render cleanly in both markdown and gh-summary
renderers (both use `if f.file else "-"`).

Signed-off-by: fOuttaMyPaint <tmhospitalitystrategies@gmail.com>
Signed-off-by: fOuttaMyPaint <154358121+TMHSDigital@users.noreply.github.com>