Fix browser pairing instructions

The first-run guide told readers to open the pairing URL in the browser address bar. In two isolated browser contexts, this landed on an empty host list. Pairing through Hosts → Add host → Paste pairing link connected successfully and displayed the existing conversation.

Updates the README to that explicit flow. Verified against the live Paseo web app and a newly provisioned Vercel Sandbox: Codex completed the fixture, a browser-originated follow-up reran the test with one pass and no edits, and stop/resume/export/cleanup passed. Documentation checks pass. No runtime changes.