feat: add github stars integration on extensions core

[JSRRosenbaum]

Summary

• add the GitHub Stars extension on top of the HN-enabled core stack
• preserve the one-app / many-sources CLI feel while inheriting the lower HN improvements

What changed

• add the gh-stars namespace and associated sync, search, list, stats, classify, distribution, markdown export, and path flows
• preserve stars storage internals while exposing the CLI namespace as gh-stars
• inherit HN Firefox username inference and refreshed HN docs/help from the lower branch
• include the bounded namespace-helper and comment/header cleanup already verified on this branch

Verification

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

Notes

• base branch: feat/hn-on-core
• reviewer focus should be the stars feature layer, not the inherited HN/core plumbing