Update build CI to trigger on comment. Free up additional runner disk space at start of workflow.

.github/workflows/build.yml

@@ -1,11 +1,29 @@
 name: "Build"
 
 on:
-  pull_request:
-    branches: [ main ]
+  issue_comment:
+    types: [created]
 
 jobs:
+  trigger-comment:
+    if: github.event.issue.pull_request && contains(github.event.comment.body, '/build')
+    runs-on: ubuntu-latest
+    steps:
+    - name: "Post trigger comment"
+      uses: actions/github-script@v7
+      with:
+        script: |
+          const workflowUrl = `${context.payload.repository.html_url}/actions/runs/${context.runId}`;
+          github.rest.issues.createComment({
+            issue_number: context.issue.number,
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            body: `🚀 Build workflow triggered! [View run](${workflowUrl})`
+          });
+
   build:
+    needs: trigger-comment
+    if: github.event.issue.pull_request && contains(github.event.comment.body, '/build')
     strategy:
       matrix:
         include:
@@ -22,8 +40,27 @@ jobs:
       image: ${{ matrix.image }}
 
     steps:
-    - name: "Checkout code"
+    - name: "Free disk space"
+      run: |
+        sudo rm -rf /usr/local/lib/android || true
+        sudo rm -rf /usr/share/dotnet || true
+
+    - name: "Retrieve PR info"
+      uses: actions/github-script@v7
+      id: pr-info
+      with:
+        script: |
+          const pr = await github.rest.pulls.get({
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            pull_number: context.issue.number
+          });
+          core.setOutput('sha', pr.data.head.sha);
+
+    - name: "Checkout PR code"
       uses: actions/checkout@v4
+      with:
+        ref: ${{ steps.pr-info.outputs.sha }}
 
     - name: "Full build with NVSHMEM"
       run: |
@@ -38,3 +75,26 @@ jobs:
         cd build-no-nvshmem
         cmake ..
         make -j$(nproc)
+
+  result-comment:
+    needs: build
+    if: always() && github.event.issue.pull_request && contains(github.event.comment.body, '/build')
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Post result comment"
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const workflowUrl = `${context.payload.repository.html_url}/actions/runs/${context.runId}`;
+            const success = '${{ needs.build.result }}' === 'success';
+
+            const message = success
+              ? `✅ Build workflow passed! [View run](${workflowUrl})`
+              : `❌ Build workflow failed! [View run](${workflowUrl})`;
+
+            github.rest.issues.createComment({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: message
+            });