ci: the e2e selectors miss Chainsaw step-file suites and loose files under examples/backups #4630

The e2e test selectors (hack/select-e2e.sh, hack/select-install.sh) have two blind spots nobody hits today, and a comment that describes discovery too narrowly.

• Chainsaw v0.2.15 treats a directory with no chainsaw-test.* but with NN-*.yaml step files as a suite (TryFindStepFiles in pkg/discovery), and discovery recurses into subdirectories. Neither selector lists such a suite, so under a narrowed selection it would never run. No suite uses that layout yet.
• A file placed directly in examples/backups/ (for example a shared common.sh) is read as inert through the examples/ rule and selects nothing. No such file exists yet.
• packages/core/testing/Makefile:166 says Chainsaw discovers "every chainsaw-test.yaml". It also loads .yml.

Fix shape: make both selectors either enumerate the step-file layout or refuse it loudly, map a loose examples/backups/* file to every backup suite or the full run, and fix the comment. Each with a select-e2e test.