docs(cloud-connection): explain why RuntimeConfigPlugin keeps '' unlike its neighbor

The corrected example now has two spellings of "no cloud" two lines apart
with no stated reason: MarketplaceInstallLocalPlugin needs the explicit
'off' sentinel, RuntimeConfigPlugin keeps ''. Unexplained, a reader could
"normalise" either direction -- including changing install-local back to
'', which reproduces the exact defect this card exists to remove.

Add a one-line comment: RuntimeConfigPlugin does not re-resolve
controlPlaneUrl through resolveCloudUrl(), so '' means "stay on this
origin" here rather than "unset" -- the asymmetry is deliberate, not a
leftover copy-trap.

Part of #8355

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P7vaLs7bhBPi9m3JyzkhDj

Author: claude

packages/cloud-connection/README.md

@@ -43,6 +43,9 @@ const plugins = [
   // NOT cloud-gated: features.marketplace is derived from what is actually
   // mounted, not from this constructor call, so a cloud-less runtime reports
   // marketplace: false on its own — there is nothing here to keep in sync.
+  // `''` here, unlike its neighbor above, is correct as-is: this plugin does
+  // NOT re-resolve controlPlaneUrl through resolveCloudUrl(), so '' means
+  // "stay on this origin" rather than "unset" — do not "fix" it to 'off'.
   new RuntimeConfigPlugin({ controlPlaneUrl: '', singleEnvironment: true, installLocal: true }),
 ];
 ```