fusion-invariants-lint-was-structural-only-not-semantic

The fusion-invariants-lint.py script only verified section-header regex and taxonomy blacklist — it could confirm a skill was shaped right but not that it worked. Three semantic checks were missing: phantom-skill-ref (references to skills that don’t exist on disk), dead-docs-pointer (cross-refs pointing to non-existent files), and vault-tag hygiene (tags not matching the allowed taxonomy). All three were added to the lint script, closing the critic’s biggest gap.