ci: trim comments in experimental.yml (#1967) (#2038)

- Drop history narration, run and job ids, timing tables, restated
  YAML and duplicated rationale; keep each non-obvious why with its
  issue ref and upstream source.
- Point the step caps' evidence at the KTLO test that pins them.
- Add tests/project/test_experimental_shellcheck_1967.py, which
  shellchecks every run: block as actionlint does.

Parsed YAML is unchanged apart from comment lines in one run: block.

Author: JarryShaw