fusion-invariants-lint-structural-only-misses-semantic-validity

The fusion-invariants-lint.py script only checked structural shape (section headers, taxonomy blacklist) — it could not detect phantom skill references, dead doc pointers, or incorrect vault tags. A lint that passes 71/71 can still have ghost references and stale cross-links. Extended with three semantic checks: phantom-skill-ref, dead-docs-pointer, vault-tag hygiene.