feat: add extension extractor router

[solomonneas]

Summary

• add a background-side extractor router for extension summarize/chat flows
• add a Reddit thread extractor that fetches /comments/<post>.json asynchronously and formats post metadata plus nested comments
• preserve the shouldPreferUrlMode hard switch so media/preferUrl URLs skip new extractors and log extractor.route.preferUrlHardSwitch

Fixes #174

Verification

• pnpm -s test tests/chrome.extractor-router.test.ts tests/content.url.test.ts tests/chrome.panel-session-actions.test.ts tests/sidepanel.panel-cache.test.ts tests/chrome.chat-context.test.ts
• pnpm -s test tests/chrome.extractor-router.test.ts tests/chrome.extension-logs.test.ts tests/extension.firefox.manifest.test.ts
• pnpm -C apps/chrome-extension exec playwright test -c playwright.config.ts --project=chromium tests/sidepanel.core.spec.ts:26 --reporter=line
• pnpm -C apps/chrome-extension test:chrome (50 passed, 3 skipped)
• pnpm -C apps/chrome-extension build
• pnpm exec oxfmt --check apps/chrome-extension/src/entrypoints/background.ts apps/chrome-extension/src/entrypoints/background/extract-cache.ts apps/chrome-extension/src/entrypoints/background/panel-summarize.ts apps/chrome-extension/src/entrypoints/background/extractors/router.ts apps/chrome-extension/src/entrypoints/background/extractors/reddit-thread.ts apps/chrome-extension/src/entrypoints/background/extractors/page-readability.ts apps/chrome-extension/src/entrypoints/background/extractors/url-daemon.ts apps/chrome-extension/src/entrypoints/background/extractors/types.ts tests/chrome.extractor-router.test.ts
• git diff --check

Notes

• The first test:chrome attempt failed because the Playwright Chromium binary was missing locally. Installed it with pnpm -C apps/chrome-extension exec playwright install chromium, then reran the Chrome tests successfully.