Add Threads login-state search adapter

[congcongfu]

Summary

• add a browser-backed Threads search adapter using the logged-in web UI
• normalize visible post results with author, text, timestamp, URL, and nullable engagement counts
• add OpenSpec artifacts and focused adapter tests

Verification

• npx vitest run --project adapter clis/threads/search.test.js
• npm run dev -- validate threads/search
• npm run check:typed-error-lint
• npm run check:silent-column-drop
• npm run build-manifest
• npm run dev -- browser --workspace bound:threads verify threads/search --trace on --live --focus --strict-memory