ci: check_coverage_matrix.py accepts dead owners and skips the DSL matrix — close the holes that let the docs rot

[JArmandoAnaya]

What

scripts/check_coverage_matrix.py passed green while ~21 XML matrix rows and most DSL Exec claims went stale — three mechanical holes:

1. A closed owner satisfies rule 3 forever. A planned (p3-s4) / deferred (#23) marker is accepted as long as the sprint id has a heading in roadmap.md — it never checks whether the owner is still open. Fix: fail on markers whose sprint is delivered / issue is closed (source: gh issue list --state closed --json number at CI time, or a maintained DELIVERED list in the script).
2. The DSL matrix has no Tests column, so rule 3 (introduced by docs(roadmap): coverage matrix must record delivery state (Tests column) and p5-s4 partial exit #68, CI-enforced for the XML matrix) is skipped entirely for osc-dsl-coverage.md (check_coverage_matrix.py:109 tolerates tests_cell is None). Fix: require the column once it exists (companion docs issue adds it) and make its absence an error.
3. TEST_DIRS omits tools/scena-run/tests and tools/scena-check/tests (check_coverage_matrix.py:82-88), so CLI suites can never be cited in a matrix.

Done when

• Checker fails on any planned/deferred marker owned by a closed issue or delivered sprint
• DSL matrix Tests column enforced; tools test dirs accepted
• The checker's own doc string / matrix rule text updated; run stays in the format CI job