Before: after `npm i -g` upgrade, the next agent-browser command would
detect daemon version mismatch, kill the old daemon, spawn a fresh one,
and connect to a brand-new about:blank tab. The user's previous
navigation state was silently lost — `get url` returned about:blank
even though the user's Chrome was still on the same page.
Now: before killing the old daemon, the CLI synchronously asks it for
its current URL via the existing socket. If non-empty and not
about:blank, it's persisted to a `.restore-url` sidecar in the socket
dir. After the new daemon spawns and auto-connects, it reads the
sidecar (read-and-delete), navigates the fresh tab to the saved URL,
and prints `⚠ Restored previous URL: <url>`.
Manual `agent-browser close` does NOT write the sidecar, so a clean
shutdown won't trigger surprise navigation. The sidecar is consumed on
read regardless of whether navigation succeeded, so a stale entry
can't haunt later auto-launches.