PAIDEIA v0.9.27 — a concept-graph skill, and lints that pin the fan-out

Improvements salvaged from workflow residue, validated against the existing suite. The manifest stays pinned at 0.9.0.

• A new concept-graph skill (skills/course-builder/concept-graph.md).
• Refinements to the /graph command, reindex.py, and verify_tool.py.
• Analyze and grade spec hardening.
• Expanded contract lints (TestAnalyzeFanOut and friends) and test_reindex coverage.

220 tests pass; the 18 on-disk commands stay consistent with the manifest.