feat(contract): add testable gating contract with minimal runner

Add the gating-contract crate which formalizes the policy enforcement
contract with:

- Inputs: GatingRequest with proposal, context, and metadata
- Outputs: GatingDecision with verdict, refusal details, evaluations
- Refusal Taxonomy: 11 categories with 25+ specific refusal codes
  - Language violations (100-199)
  - Toolchain violations (200-299)
  - Security violations (300-399)
  - Pattern violations (400-499)
  - Spirit violations (500-599)
  - System errors (900-999)
- Audit Log: AuditEntry with full decision traceability
- Minimal Runner: ContractRunner and TestHarness for validation

CLI additions:
- conative contract schema: Display contract specification
- conative contract test: Run tests from training data
- conative contract eval: Evaluate gating requests with audit output

All 8 contract unit tests pass. Test harness exposes known Oracle
limitations (marker-based detection has false positives for TypeScript
and Python markers in Rust/Elixir code).

Author: claude