Add cell-structure literature discovery skill

[realmarcin]

Summary

• Add a new Claude skill for discovering CellStructureMech records from the literature
• Document candidate boundary checks, ignored/hidden duplicate searches, GO-vs-local identity selection, evidence bundles, causal-graph expectations, and validation gates
• Route reusable source adoption to source-queue and snippet/readability checks to literature-evidence
• Fix adversarial review findings around provenance flags, structure relationship semantics, and duplicate-search precision

Validation

• uv run pytest tests/test_skill_frontmatter.py
• uv run python scripts/run_qc.py
• git diff --check

Review Issues

• Closes discover-cell-structures new-record example omits curation provenance flags #189
• Closes discover-cell-structures conflates parent_structures with part_of #190
• Closes discover-cell-structures duplicate search uses bare DOI and PMID prefixes #191

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