feat(coding-standard)!: remove pr-diff feature and built-in php setup (#224)

BREAKING CHANGE: Much of the "setup" that's built-into the command is removed in favor of a leaner action. This also includes the "on PR, only diff PR contents" behavior. This can be restored, but it shouldn't be the default and should be done as an input.

Fix SEVERITY_FLAGS construction which exited 1 under bash -e when
severity inputs were empty, causing CI failures.

coding-standard/README.md

@@ -11,6 +11,8 @@ See the [action.yml](./action.yml)
 
 ## Usage
 
+The caller is responsible for checking out the repository and setting up PHP before calling this action.
+
 ```yml
 name: Coding Standard
 
@@ -26,11 +28,19 @@ jobs:
   coding-standard:
     runs-on: ubuntu-latest
     steps:
+    - uses: actions/checkout@v6
+
+    - uses: shivammathur/setup-php@v2
+      with:
+        php-version: '8.3'
+        tools: composer:v2
+        coverage: none
+
     - uses: graycoreio/github-actions-magento2/coding-standard@main
       with:
+        path: app/code # Optional, defaults to .
         version: 25 # Optional, will use the latest if omitted.
-        path: app/code # Optional, will be used when event is not a pull request.
         severity: 8 # Optional, will use phpcs default of 5 if not specified.
         warning_severity: 4 # Optional, will use severity value if not specified.
         error_severity: 7 # Optional, will use severity value if not specified.
-```
+```