Restrict ros2-ci.yml to code paths only

[frealaf]

The ros2-ci workflow runs on all pushes/PRs, even when editing only workflows or documentation.

Update the workflow to run only when code changes happen in these paths:

• src/**
• CMakeLists.txt
• package.xml

This reduces unnecessary CI runs when only workflow files or docs are updated.

Next steps:

• Verify that the restriction behaves correctly.
• Confirm that changes to .github/workflows no longer trigger ros2-ci.
• Ensure we don’t miss important CI checks on code changes.