ci: add SonarCloud scan via shared reusable workflow

Calls storytold/craft-common-actions/.github/workflows/sonarcloud.yml@main
with secrets: inherit. Adds SonarCloud badge to README where missing and
removes the redundant sonar-project.properties (config now passed by the
reusable workflow).

.github/workflows/sonarcloud.yml

@@ -0,0 +1,20 @@
+name: SonarCloud
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: sonarcloud-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  sonarcloud:
+    # Tracked at @main so org-wide CI improvements propagate without PR churn.
+    uses: storytold/craft-common-actions/.github/workflows/sonarcloud.yml@main
+    secrets: inherit

README.md

@@ -1,4 +1,8 @@
 <h1 align="center">ArtCraft</h1>
+
+<p align="center">
+  <a href="https://sonarcloud.io/project/overview?id=storytold_artcraft"><img alt="Scanned by SonarCloud" src="https://img.shields.io/badge/Scanned_by-SonarCloud-4b9fd5?style=flat-square&logo=sonarqubecloud&logoColor=white"></a>
+</p>
 <p align="center"><strong>The IDE for artists.</strong></p>
 
 <!-- Main video: https://www.youtube.com/watch?v=kzvQMdg66Go