[EPIC] SAP Metadata Coordination Pilot

## What
Implement a coordination workflow between Data.gov and ResearchDataGov.org (the Standard Application Process portal) so that:
- Agencies can satisfy both systems with a single metadata submission, and the SAP portal harvests directly from Data.gov
- Public users discovering restricted datasets in catalog.data.gov have a clear, actionable path to apply for access through the SAP

## Why
OMB M-25-05 (January 2025) explicitly directs that Data.gov serve as the authoritative metadata source for the SAP, and that "agencies need only create and maintain the comprehensive data inventory required under this Memorandum to satisfy the requirements of both Titles II and III of the Evidence Act." Currently, agencies must submit metadata to both systems separately. This epic covers the discovery, design, and pilot implementation of both the metadata/harvest integration and the catalog UI treatment for SAP-flagged datasets.

## Done When
- A near-term pilot approach is agreed with the SAP PMO
- A metadata flag mechanism is defined and documented
- At least one agency is onboarded through the pilot workflow
- The SAP portal is able to harvest a filtered subset of Data.gov metadata
- catalog.data.gov displays a notice and application link on SAP-flagged dataset records
- Pilot results are documented with a scale-up recommendation

## Notes
- Policy anchor: M-25-05, Section 5(b)(v)
- Relevant DCAT-US 3.0 fields: `accessRights` (access level signal) plus a SAP-specific flag (field TBD)
- DCAT-US 3.0 rollout is an upstream dependency for the catalog UI work -- tracked separately
- Contractor bandwidth window closes September 2026 -- pilot should be scoped to fit