Document abstract reasoning gate semantics for validation-time enforcement

[mdheller]

This PR adds the runtime-governance note for the abstract reasoning gate.

Why:

• the abstract lane is now defined in policy, benchmark, and semantic-artifact PRs, but Agentplane still needs an explicit validation-time enforcement spec
• abstract and program-induction branches should not proceed on llm-only posture
• the next code patch in evaluate_control_matrix_gate.py and validate_bundle.py needs a repo-native front door

What this PR adds:

• abstract reasoning gate v0 spec note under runtime governance
• intended bundle policy shape
• intended denial conditions
• intended ControlGateArtifact and ValidationArtifact exposure

What this PR does not do:

• it does not yet patch the Python gate implementation
• it does not yet extend execution artifacts