data: add groundcover startup offer

[itzroberl]

Reserves #1259

What changed

Adds one new groundcover entity and one current offer for eligible AWS startup customers: 25% off the first-year subscription through AWS Marketplace. The record models AWS Startups as the access operator and leaves the underlying subscription price unknown.

Sources

• https://startups.aws.com/offers?lang=en-US (search for "groundcover for Startups")
• https://www.groundcover.com/aws-marketplace
• https://docs.groundcover.com/

Validation

• Official local sourcey-catalog-verify preflight: passed
• git diff --check: passed

Certification

• I changed only allowed repository content and did not mix Entity YAML with other paths.
• Public sources substantiate every submitted factual value; any Sourcey-written prose is a neutral summary, not a fabricated vendor quote.
• I did not author verification, provenance, freshness, or signature claims.
• My commits include a Signed-off-by line.