Rebrand workflow as Tabellio

[hudsonaikins]

Summary

• Rebrand the open-source workflow from Evident to Tabellio across README, docs, package metadata, schemas, examples, workflow, and scripts.
• Rename the reusable workflow, script names, evidence artifact, schema version, and fixture paths to Tabellio.
• Replace the README hero with a generated Tabellio product image and add a generated Tabellio logo asset.
• Update NOTICE copyright naming and GitHub repository metadata/topics.

Validation

• npm run check
• node --check scripts/check-tabellio-evidence-envelope.mjs && node --check scripts/check-tabellio-external-actions.mjs && node --check scripts/write-tabellio-evidence-envelope.mjs
• actionlint
• npm pack --dry-run --json
• git diff --check