Re-enable full install test with extended timeout

- Test runs on PRs and manual workflow_dispatch
- Increased timeout from 45 to 90 minutes for source builds
- Validates end-to-end functionality
- Can add caching in future to speed up dependency builds

Test breakdown:
- Lint & Spec: ~5 min (every push)
- Install test: ~70 min (PRs only)

Author: sbryngelson

.github/workflows/spack.yml

@@ -112,9 +112,9 @@ jobs:
 
   test-install:
     name: Test Installation & Execution
-    # Disabled by default - takes too long (60+ min building from source)
-    # Enable manually via workflow_dispatch when needed
-    if: github.event_name == 'workflow_dispatch'
+    # Only run on pull requests or manual workflow dispatch
+    # Note: Takes 60+ min due to building dependencies from source
+    if: github.event_name == 'pull_request' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout MFC
@@ -164,7 +164,7 @@ jobs:
           # Install minimal configuration
           # Binary cache is already configured, Spack will use it automatically when available
           spack install --show-log-on-error mfc~mpi~post_process
-        timeout-minutes: 45
+        timeout-minutes: 90
 
       - name: Test MFC Execution
         run: |