ci: format code on pull requests and run only on PRs

[JTorkk]

Problem

The EndBug/add-and-commit step failed:

Error: fatal: You are not currently on a branch.
To push the history leading to the current (detached HEAD) state now, use
    git push origin HEAD:<name-of-remote-branch>

actions/checkout defaults to a detached HEAD, and on pull_request events it checks out the merge ref rather than the PR's source branch. So there was no branch for the formatting commit to be pushed to.

Fix

• Check the branch out by name with ref: ${{ github.head_ref }} so the commit step has a real branch to push to
• Grant the job permissions: contents: write so it is allowed to push
• Guard the commit step with if: github.event_name == 'pull_request' — workflow_dispatch runs have no head_ref and nothing to push to
• Bump actions/checkout v3 → v4 (v3 runs on a Node version GitHub is deprecating)

Formatting is still applied in place by dart format . and committed back to the PR branch, so the package's style stays consistent automatically.

Also: CI now runs on pull requests only

The push-to-main trigger is removed, as requested.

Worth being explicit about the tradeoff, since it is exactly how the bug in #9 shipped: c915fe5 ("chore: update project dependencies") was pushed straight to main, so CI never ran on it — even though the existing round-trip tests would have caught the regression immediately. With PR-only triggers that gap remains open unless main is protected.

Recommended: enable a branch protection rule on main requiring a PR and a passing [CI] check. That gets the same guarantee without running CI on pushes.

Verified locally

flutter pub get                     OK
dart format .                       Formatted 47 files (0 changed)
flutter analyze . --fatal-infos     No issues found!
flutter test --no-pub               +132: All tests passed!