chore: drop @objectstack/service-cloud from changeset fixed list (#1261)

The package was removed from this repo in #1257 (cloud split) but its
entry in .changeset/config.json's `fixed` list was left behind. Every
Release workflow run since then has failed with:

  ValidationError: The package or glob expression
  "@objectstack/service-cloud" specified in the `fixed` option does
  not match any package in the project.

Removing the entry unblocks 'changesets/action', which will then update
PR #1256 (chore: version packages) to bump remaining published packages
4.0.5 -> 4.1.0 (minor, driven by the @objectstack/cli changeset added
in #1260). Merging that PR triggers npm publish.

Co-authored-by: Jack Zhuang <50353452+hotlong@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.changeset/config.json

@@ -45,7 +45,6 @@
       "@objectstack/service-realtime",
       "@objectstack/service-ai",
       "@objectstack/service-storage",
-      "@objectstack/service-cloud",
       "@objectstack/service-package",
       "@objectstack/service-tenant",
       "@objectstack/docs",