feat: add hacker news integration on extensions core

[JSRRosenbaum]

Summary

• add the Hacker News extension on top of feat/extensions-core
• include the latest HN auth/status UX improvements and Firefox username inference

What changed

• add HN sync, status, stats, search, list, classify, distribution, markdown export, and path commands
• add HN-specific paths on top of the shared extensions core helpers
• make ft hn sync infer the HN username from Firefox when --username is omitted
• refresh HN docs/help text to match the new sync behavior

Verification

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

Notes

• explicit --username still works and overrides inference
• this PR should be reviewed on top of feat/extensions-core