data: add Plivo Startup Credits offer

[luizguilhermetatsch]

What changed

Adds one new vendor record for the Sourcey startup-credits catalog:

Plivo — vendors/pl/plivo.yaml

• New vendor (slug: plivo), new program (plivo-startup-credits-program), new offer (plivo-startup-credits).
• The offer: startups backed by a Plivo partner fund (Y Combinator, Techstars, Antler, Lightspeed, Initialized Capital, Boldstart Ventures, South Park Commons, and others) can claim Plivo platform credits for voice, SMS, WhatsApp, and chat services.
• Application is a public form on the first-party page; a Plivo account is required.
• Source: https://www.plivo.com/startups/

Verification

Validated locally against the digest-pinned Sourcey Candidate Verifier
(sourcey-candidate-verifier.js validate-change) with the pinned taxonomy:

• Individual run (base f74092c → head 991b6d8): {"status":"valid","vendors":1,"programs":1,"offers":1}

Certification

• I changed only allowed repository content and did not mix vendor YAML with other paths.
• I did not author verification, provenance, freshness, or signature claims.
• My commits include a Signed-off-by line.