ci(release): standardize production workflows on main

Route production builds, releases, and changelog generation through `main`, update contribution guidance for main/next branch targets, and cover the branch policy in workflow tests.

.github/workflows/coolify-release.yml

@@ -30,9 +30,9 @@ jobs:
       version: ${{ steps.version.outputs.version }}
     steps:
       - name: Reject releases outside the production branch
-        if: ${{ ! contains(fromJSON('["v4.x", "main"]'), github.ref_name) }}
+        if: ${{ github.ref_name != 'main' }}
         run: |
-          echo "Stable releases must run from v4.x or main, not ${{ github.ref_name }}."
+          echo "Stable releases must run from main, not ${{ github.ref_name }}."
           exit 1
 
       - uses: actions/checkout@v5