data: add Warestack startup offer - #1437

What changed

• add Warestack as one new canonical Entity record
• model its active startup-specific offer: the Starter plan free for six months, with published limits and savings of up to $2,000
• encode eligibility, work-email requirement, application route, duration, features, and economics from Warestack's first-party English-language pages

Sources

• https://www.warestack.com/startup-program
• https://www.warestack.com/

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.

Validation

• pinned public Sourcey verifier: passed (1 entity, 1 program, 1 offer)
• git diff origin/main...HEAD --check: passed
• DCO sign-off: included

Closes #1436. Duplicate checks before reservation found no live Warestack entity or competing contribution.

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