Fix KasmVNC QuPath CI and branch isolation

[FloWuenne]

Summary

• Pin all GitHub Actions workflow dependencies to full-length commit SHAs.
• Scope PR/release workflows to the single KasmVNC QuPath Studio.
• Remove leftover non-QuPath example files and the duplicate kasmVNC/qupath/Dockerfile, leaving .seqera/Dockerfile as the only Dockerfile.

Testing

• Verified every workflow uses: reference is pinned to a 40-character SHA.
• Verified .seqera/Dockerfile is the only Dockerfile in the branch.
• Parsed the workflow and Studio YAML files with PyYAML.
• Confirmed GitHub Actions Build PR Images / detect-changes passed on the stacked PR.
• Docker image build could not be run locally because this VM does not provide Docker or another container builder.