ci: name change-detection and test-discovery steps for readability

[MGPOCKY]

Problem

In .github/workflows/ci.yml, several helper run steps are unnamed and longer than 300 characters. In the GitHub Actions UI those steps collapse into generic labels, which hurts readability when UI-only change detection or test matrix discovery fails.

Changes

Semantics are unchanged: same step ids, same GITHUB_OUTPUT keys, and the same job output / matrix wiring.

• Add name: Detect UI-only changes to id: check
• Add name: Discover VM test files to id: find in discover-vm-tests
• Add name: Discover integration test files to id: find in discover-integration-tests

Test plan

• Confirm later jobs still use needs.detect-changes.outputs.ui-only
• Confirm VM / integration matrices still use needs.*.outputs.tests
• Confirm the steps show the new names in the Actions UI