refactor: type pipeline core

[jackwener]

Summary

• tighten typing across the pipeline executor, template engine, and common step handlers
• replace broad any usage in browser/fetch/transform steps with explicit unknown + narrowing
• adjust transform tests to assert typed results cleanly

Validation

• npm run typecheck
• npm run build
• npx vitest run src/pipeline/template.test.ts src/pipeline/transform.test.ts src/pipeline/executor.test.ts