Skip wheel/sdist builds on PRs - only run unit tests

PR runs: 7 jobs (unit_tests only)
Main/tag runs: 7 + 49 + 1 = 57 jobs (full build)

Reduces PR CI from 57 to 7 jobs (88% reduction)

Author: claude

.github/workflows/cibuildwheel.yml

@@ -53,6 +53,8 @@ jobs:
         run: pytest tests -vv
 
   build_wheels:
+    # Skip wheel builds on PRs - only build on main branch and tags
+    if: github.event_name != 'pull_request'
     name: Build wheels on ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
     timeout-minutes: 90
@@ -342,6 +344,8 @@ jobs:
           overwrite: true
 
   build_sdist:
+    # Skip sdist builds on PRs - only build on main branch and tags
+    if: github.event_name != 'pull_request'
     name: Build source distribution
     runs-on: ubuntu-latest
     steps: