feat: add reddit integration on extensions core

[JSRRosenbaum]

Summary

• add the Reddit extension on top of the stars-enabled stack
• keep the CLI aligned with the shared multi-source structure while preserving Reddit-specific behavior

What changed

• add the reddit namespace and associated sync, status, search, list, stats, classify, distribution, markdown export, and path flows
• align Reddit status rendering with the shared status structure
• port the extracted CLI registration pattern onto this branch
• inherit HN Firefox username inference and refreshed HN docs/help from lower branches

Verification

• npx tsx --test tests/hn-cli.test.ts tests/reddit-cli.test.ts tests/stars-cli.test.ts
• npm run build

Notes

• base branch: feat/gh-stars-on-core
• reviewer focus should be the Reddit feature layer plus the local registrar extraction on this branch