Tasks

• [x] Specify shared ABC, concrete-provider scope, and verification plan.
• [x] Verify candidates and document TODOs for unavailable endpoints.
• [x] Add synthetic multi-provider behavior regressions.
• [x] Extract shared UseDirect models and ABC; preserve California's behavior.
• [x] Add concrete providers, seed migration, and registry-driven metadata CLI help.
• [x] Verify worker ingestion, provider isolation, and migration round trips.
• [x] Run lint, type checks, tests, and pre-commit hooks.
• [x] Update documentation, feature checklist, and validation evidence.