fix: repair clang-format CI for fork PRs and push events

[ibell]

Summary

Fixes #2724 — the dev_clangformat workflow was silently passing on the majority of PRs.

• Fork PRs: replaced remotes/origin/$GITHUB_HEAD_REF / remotes/origin/$GITHUB_BASE_REF with commit SHAs from the GitHub Actions context (github.event.pull_request.base.sha and github.sha). The fork's branch is never fetched into origin, so the old ref always resolved to nothing and git diff reported 0 changed files.
• Push events: GITHUB_HEAD_REF and GITHUB_BASE_REF are only populated for pull_request events, not push. The fix detects the event type and uses github.event.before / github.sha for push events.
• Action versions: bumped actions/checkout v3 → v6 and actions/upload-artifact v4 → v7 (both at latest stable).
• Removed redundant git fetch --all (not needed since fetch-depth: 0 already fetches everything and we now use SHAs).

Test plan

• Open a PR from a fork branch and confirm the workflow now correctly identifies changed .cpp/.h files
• Verify a push to master/develop runs the diff against github.event.before
• Confirm a correctly-formatted PR still passes (exit 0)
• Confirm a poorly-formatted PR fails and uploads the patch artifact

🤖 Generated with Claude Code

[henningjp]

Nice. Verified PR behavior from forked repos using #2732 draft PR.

• Looked like it passed (no artifacts generated)

So, I intentionally messed with the code format in another commit.

• It successfully failed the clang-format test.
• It successfully uploaded the clang-format repair artifact.

I don't know how to check the second Test Plan item from master/develop branch.