Improve ONNX external data runtime evidence gates

[MAUROCERON]

Implements #1385.

Summary

• Adds an ONNX External Data and Runtime Provider Evidence Gate to model-supply-chain.
• Requires full ONNX/ORT bundle manifests, external tensor digest binding, path containment checks, conversion/opset provenance, parity testing, runtime provider policy, fallback evidence, and custom operator/provider provenance.
• Adds static edge-case fixtures for external-data bundle gaps, conversion parity gaps, provider fallback, benign complete evidence, and Not Evaluable handling.

Validation

• Markdown fence balance checked for SKILL.md and tests/onnx-external-data-runtime-edge-cases.md.
• Marker checks passed for ONNX bundle, external data, path containment, conversion provenance, runtime provider policy, fallback, and Not Evaluable scenarios.
• Official reference URLs returned HTTP 200:
  • ONNX External Data Security
  • ONNX External Data
  • ONNX Runtime docs
  • ONNX Runtime Execution Providers
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