Hide verifier and attestor internals from SDK root exports

[signerless]

Summary

• remove the public verifier program constant and task attestor seed from the SDK root surface
• stop exporting the task attestor PDA helper from the root package and keep the PDA seed private inside task internals
• update public API tests and the API baseline to reflect the trimmed SDK contract

Validation

• npm run typecheck
• npm test
• npm run build
• npm run api:baseline:check