Add Legal Core runtime action admission gate

[mdheller]

Summary

Adds the AgentPlane Legal Core runtime action-admission tranche.

This creates:

• schemas/legal-core/runtime-action-admission-legal-gate.v0.1.schema.json
• docs/legal-core-runtime-gate-v0.1.md
• tests/fixtures/legal-core/runtime-action-admission.static-doc-triage.allowed.json
• tests/fixtures/legal-core/runtime-action-admission.unknown-authority.denied.json

Why

AgentPlane is the governed Act boundary for the World Evidence Fabric. Runtime action admission needs to reference Legal Core decisions before sandbox allocation, runtime execution, and evidence promotion.

This PR makes the first runtime gate explicit:

• every action proposal binds to a legal-core-decision:* ref;
• unknown authority denies runtime action and sandbox allocation;
• safe-processing blocks unsafe rendering and credential access by schema;
• sandbox allocation does not imply execution;
• execution does not imply production parity;
• evidence promotion requires Legal Core and claim-boundary receipts;
• denial receipts are first-class evidence;
• non-claims are mandatory.

Validation

Static schema/fixture tranche only in this PR.

Manual structural checks performed:

• branch is ahead of main by 4 commits;
• added schema and fixtures are JSON documents;
• allowed fixture admits read-only static document triage but blocks evidence promotion;
• denied fixture blocks unknown authority before runtime action, sandbox allocation, and evidence promotion.

Non-goals

• Does not provide jurisdiction-specific legal advice.
• Does not certify production readiness.
• Does not let sandbox allocation imply infrastructure execution.
• Does not let runtime execution imply production parity.
• Does not promote runtime output into SynapseIQ, Sherlock, Holmes, MeshRush, CairnPath, GAIA, Ontogenesis, or Sociosphere without Legal Core and claim-boundary receipts.

Related

• Legal Core runtime gate for AgentPlane action admission and receipts #266
• Reference architecture rollout: canonical governed-intelligence contract sociosphere#310
• Add Legal Core v0.1 policy kernel fixtures SCOPE-D#62