feat: fix duplicate CI runs on pull requests

[Copilot]

PRs from branches in the same repository trigger both push and pull_request events, causing duplicate workflow runs.

Changes

Modified .github/workflows/main.yml to restrict push triggers to the main branch only:

# Before
on: [push, pull_request]

# After
on:
  push:
    branches:
      - main
  pull_request:

This ensures:

• PRs run once on pull_request events
• Main branch runs once on push events (post-merge)
• semantic-release job continues to work correctly

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[bug] needs to fix CI for strange duplicated checks in PR</issue_title>
<issue_description>The following duplicated CI checks ran in this open PR: #508

failing checks
[main / test (^18) (push)](https://github.com/casbin/node-casbin/actions/runs/19272050250/job/55102857598?pr=508)
main / test (^18) (push)Failing after 9s
[main / test (^20) (pull_request)](https://github.com/casbin/node-casbin/actions/runs/19272051183/job/55102867257?pr=508)
main / test (^20) (pull_request)Failing after 4s
[main / test (^20) (push)](https://github.com/casbin/node-casbin/actions/runs/19272050250/job/55102857620?pr=508)
main / test (^20) (push)Failing after 5s
[main / test (^22) (pull_request)](https://github.com/casbin/node-casbin/actions/runs/19272051183/job/55102867274?pr=508)
main / test (^22) (pull_request)Failing after 9s

I think for an open state PR, it should not run "XXX (push)" checks, right? It should run for the git push after PR merge?
Need to fix the CI.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [bug] needs to fix CI for strange duplicated checks in PR #512

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