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-sha-build.yml

@@ -2,7 +2,7 @@ name: Build Coolify (SHA)
 
 on:
   push:
-    branches: ["v4.x", "main"]
+    branches: ["main"]
 
 permissions:
   contents: read