feat(cli): non-blocking 'update available' notice so users know to upgrade

The CLI ships as a GitHub Release binary with a manual `chrome-use upgrade`,
but nothing told users a newer version existed — so releases didn't reach them.

Add a lightweight update check: each run reads a cached latest-version and, if
it's newer than the running binary, prints a one-line hint to STDERR (never
stdout, so --json stays clean): "⚠ chrome-use X.Y.Z is available — run
chrome-use upgrade". The cache is refreshed at most once a day by a DETACHED
`__update-check` worker (curl → GitHub latest release), so the current command
never waits on the network. Skipped for meta commands (upgrade/install/doctor/
__*/--version/--help), in CI, in daemon mode, and via
CHROME_USE_NO_UPDATE_CHECK / AGENT_BROWSER_NO_UPDATE_CHECK.

Verified: nag shows for a newer cached version, suppressed by the opt-out env +
on meta commands + when up-to-date; the detached worker writes the real latest
tag from the GitHub API.

cli/src/main.rs

@@ -586,6 +586,17 @@ fn main() {
         return;
     }
 
+    // Hidden update-check worker, spawned detached by maybe_notify_update() to
+    // refresh the cached latest version without blocking a real command.
+    if env::args().nth(1).as_deref() == Some("__update-check") {
+        upgrade::run_update_check();
+        return;
+    }
+
+    // Non-blocking "update available" hint (stderr only; self-skips meta
+    // commands, daemon mode, CI, and the opt-out env vars).
+    upgrade::maybe_notify_update();
+
     // Native daemon mode: when AGENT_BROWSER_DAEMON is set, run as the daemon process
     if env::var("AGENT_BROWSER_DAEMON").is_ok() {
         // Ignore SIGPIPE so the daemon isn't killed when the parent drops