Add math-olympiad skill — adversarial verification for competition math

The skill that addresses the Proof-or-Bluff gap: self-verified 85.7% IMO
becomes <5% under human grading. Uses fresh-context verifiers armed with
specific failure patterns (not generic 'check logic').

Validated: 17/18 IMO+Putnam 2025 solved, 0 false positives, 2 novel proofs.
See eval data in anthropic monorepo sandbox/sandbox/ralph/math_skills/.

plugins/math-olympiad/README.md

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+# math-olympiad
+
+Competition math solver with adversarial verification.
+
+## The problem
+
+Self-verification gets fooled. A verifier that sees the reasoning is biased toward agreement. arXiv:2503.21934 ("Proof or Bluff") showed 85.7% self-verified IMO success drops to <5% under human grading.
+
+## The approach
+
+- **Context-isolated verification**: verifier sees only the clean proof, never the reasoning trace
+- **Pattern-armed adversarial checks**: not "is this correct?" but "does this accidentally prove RH?" / "extract the general lemma, find a 2×2 counterexample"
+- **Calibrated abstention**: says "no confident solution" rather than bluff
+- **Presentation pass**: produces clean LaTeX/PDF after verification passes
+
+## Validation
+
+17/18 IMO+Putnam 2025 problems solved, 0 false positives, 2 novel proofs found. See the skill's eval data in the [anthropic monorepo](https://github.com/anthropics/anthropic/tree/staging/sandbox/sandbox/ralph/math_skills/eval_harness).
+
+## Install
+
+```
+/plugin install math-olympiad@claude-plugins-official
+```
+
+## Use
+
+```
+> Solve this IMO problem: [statement]
+```
+
+The skill auto-triggers on "IMO", "Putnam", "olympiad", "verify this proof", etc.