docs: update Workflow SDK branding

Summary

• replace active Workflow DevKit branding with Workflow SDK in the README and workflow builder UI
• update product and generated documentation links to canonical workflow-sdk.dev URLs

Test plan

• pnpm discover-plugins
• pnpm type-check
• pnpm dlx @biomejs/biome@2.3.4 check --write --no-errors-on-unmatched README.md \"app/api/workflows/[workflowId]/download/route.ts\" \"components/workflow/nodes/add-node.tsx\"
• scan active source for retired names and domains
• pnpm fix (the repository script resolves incompatible latest Ultracite/Biome tooling)

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