ci: run CI checks on push to main

Add push trigger for main branch so CI validates the post-merge
state. Skip the PR title lint job on push events since it only
applies to pull requests.

.github/workflows/ci.yml

@@ -2,6 +2,8 @@ name: CI
 run-name: CI — ${{ github.head_ref || github.ref_name }}
 
 on:
+  push:
+    branches: [main]
   pull_request:
     branches: [main]
   # Allow running manually for debugging.
@@ -23,6 +25,7 @@ jobs:
   # ──────────────────────────────────────────────
   commitlint:
     name: Lint PR Title
+    if: github.event_name == 'pull_request'
     runs-on: ubuntu-latest
     steps:
       - uses: amannn/action-semantic-pull-request@v5