feat: add web integration on extensions core

[JSRRosenbaum]

Summary

• add the Web extension on top of the stars-enabled stack
• finish the CLI registration split so the integrated multi-source CLI is easier to review and maintain

What changed

• add the web namespace and associated sync, status, search, list, stats, classify, fetch, simplify, export, distribution, markdown export, and path flows
• extract CLI registration into focused modules:
  • src/cli-context.ts
  • src/register-hn-commands.ts
  • src/register-stars-commands.ts
  • src/register-web-commands.ts
  • src/register-wiki-commands.ts
  • src/register-utility-commands.ts
• reduce src/cli.ts to a clearer composition root
• add root registration coverage tests
• include the HN Firefox username inference and refreshed HN docs/help inherited through the stack

Verification

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

Notes

• base branch: feat/gh-stars-on-core
• reviewer focus should be the web feature layer plus the CLI registrar split