CLI: support reading HTML from stdin

[Jkker]

Summary

• allow defuddle parse to read HTML from stdin when no source is provided or when - is passed explicitly
• refactor CLI parsing into testable helpers while preserving existing file and URL behavior
• document stdin usage and add focused CLI coverage for stdin, file input, and interactive-stdin errors

Testing

• npm test
• npm test -- tests/cli.test.ts
• npm run build:node
• node dist/cli.js parse tests/fixtures/general--appendix-heading.html --property title
• cat tests/fixtures/general--appendix-heading.html | node dist/cli.js parse --property title
• node dist/cli.js parse https://stephango.com/saw --property title
• curl -sSL https://stephango.com/saw | node dist/cli.js parse - --property title