ci: prefix PR workflow steps and use Agent runner

- rename PR workflow and steps with 'PR -' prefix
- switch PR jobs and build job to Agent runner label

.gitea/workflows/build.yml

@@ -11,7 +11,7 @@ env:
 
 jobs:
   build:
-    runs-on: linux
+    runs-on: Agent
     steps:
       - name: Build Docker image
         run: |

.gitea/workflows/pr-validation.yml

@@ -1,4 +1,4 @@
-name: Pull Request Validation
+name: PR - Pull Request Validation
 
 on:
   pull_request:
@@ -6,9 +6,9 @@ on:
 
 jobs:
   validate-pr-title:
-    runs-on: linux
+    runs-on: Agent
     steps:
-      - name: Validate PR title follows Conventional Commits
+      - name: PR - Validate PR title follows Conventional Commits
         run: |
           PR_TITLE="${{ github.event.pull_request.title }}"
           echo "Validating PR title: $PR_TITLE"
@@ -47,9 +47,9 @@ jobs:
           fi
 
   validate-docker:
-    runs-on: linux
+    runs-on: Agent
     steps:
-      - name: Validate Dockerfile syntax
+      - name: PR - Validate Dockerfile syntax
         run: |
           if [ -f "Dockerfile" ]; then
             echo "✅ Dockerfile exists"
@@ -61,7 +61,7 @@ jobs:
             exit 1
           fi
 
-      - name: Check .dockerignore exists
+      - name: PR - Check .dockerignore exists
         run: |
           if [ -f ".dockerignore" ]; then
             echo "✅ .dockerignore exists"
@@ -69,7 +69,7 @@ jobs:
             echo "⚠️  Warning: .dockerignore not found (recommended)"
           fi
 
-      - name: Validate nginx config
+      - name: PR - Validate nginx config
         run: |
           if [ -f "nginx.conf" ]; then
             echo "✅ nginx.conf exists"
@@ -81,9 +81,9 @@ jobs:
           fi
 
   build-test:
-    runs-on: linux
+    runs-on: Agent
     steps:
-      - name: Build and test Docker image
+      - name: PR - Build and test Docker image
         run: |
           docker build -t karaokepedia:test .
           CONTAINER_ID=$(docker run -d -p 8080:80 karaokepedia:test)
@@ -125,9 +125,9 @@ jobs:
           echo "✅ All container tests passed"
 
   check-files:
-    runs-on: linux
+    runs-on: Agent
     steps:
-      - name: Check required files exist
+      - name: PR - Check required files exist
         run: |
           REQUIRED_FILES=(
             "karaoke.karaniwan.org/index.html"
@@ -158,7 +158,7 @@ jobs:
           echo ""
           echo "✅ All required files present"
 
-      - name: Check for HTTrack artifacts in content
+      - name: PR - Check for HTTrack artifacts in content
         run: |
           echo "Checking that HTTrack artifacts are not included in Docker context..."