ci: run pull-request checks for every base branch (#7545)

Stacked PRs and PRs into next got no CI because the pull_request trigger
was limited to main. Drop the base filter from CI and desktop packaging,
and lock it with a workflow contract test.

.github/workflows/ci.yml

@@ -3,8 +3,8 @@ name: CI
 on:
   push:
     branches: [main]
+  # Every base branch, so stacked PRs and PRs into `next` get the same checks.
   pull_request:
-    branches: [main]
   merge_group:
   workflow_dispatch:
 

.github/workflows/desktop-packages.yml

@@ -6,7 +6,6 @@ on:
   # Packaging leaves ordinary PR CI, but a PR that changes what gets packaged
   # or how it is smoked still proves the real artifacts before merge.
   pull_request:
-    branches: [main]
     paths:
       - "packages/desktop/**"
       - "plugins/**"