SAP coordination: define metadata flag for SAP-eligible datasets

### What
Identify and document the specific DCAT-US 3.0 metadata field and value that agencies will use to signal that a dataset is accessible through the SAP, and that the SAP portal will use as a harvest filter.

### Why
The harvest approach requires a reliable, standardized signal in the metadata record. We need to agree on the field before agencies can start using it or the SAP portal can filter on it.

### Done When
- [ ] A specific DCAT-US 3.0 field is identified as the SAP flag (e.g., `accessRights`, `rights`, or other)
- [ ] The value or pattern to use is documented with an example record
- [ ] James Brown (jbrown-xentity) has been consulted to confirm the approach is schema-compatible
- [ ] The flag definition is approved by Sue and shared with the SAP PMO
- [ ] A short guidance note for agencies is drafted (can be incorporated into resources.data.gov later)

### Notes
- DCAT-US 3.0 `accessRights` field is the most likely candidate -- it replaces `accessLevel` from v1.1
- Do not introduce a non-standard field without checking with James first
- The flag needs to be something agencies can set in their data.json inventory without a large lift
- This ticket is a blocker for the harvest filter and pilot onboarding tickets