Add installed-plugin proof infrastructure

[uncfreak1255-code]

What changed

• add a read-only Claude installed-plugin probe with fail-closed event verification
• cover missing, tampered, escaped, and reordered access evidence with focused tests
• allow benchmark comparisons against a frozen prior skill, including sealed runs
• keep the LoopSpine skill, dogfood records, adaptive behavior, and global configuration unchanged

Proof

• npm test
• claude plugin validate .
• npm run probe:installed-plugin
• one-sample sealed baseline-path smoke confirmed both skill hashes were frozen v0.2.0; no performance claim is made
• Codex autoreview: clean

Boundaries

• no adaptive trigger or reference
• no hooks or global changes
• no benchmark corpus or scorer changes
• no generated probe receipts promoted