fix: review cleanup — dead code, regex consolidation, unreachable fix

[CodeSigils]

5 cleanup fixes from code review:

1. Remove dead contains_markdown_phrase() from _common.py (never called)
2. Consolidate SKILL_REF_RE — import from _common in cron-health.py instead of duplicating
3. Fix misspelled AssertionError → RuntimeError in _url_contract.py:127
4. Simplify version check — replace over-complex most_common() with direct comparison
5. (automatic) Remove redundant import re as _re inside deleted function

Net: -14 lines. All 35 tests pass, ruff clean, LSP clean.