feat(action-gate): deterministic Gate/Prove policy firewall & hash-chained ledger for MCP tool execution

[AAH20]

Summary

Adds action-gate, an MCP security middleware server implementing deterministic Gate/Prove execution boundaries, Simulation Mode, and append-only hash-chained action ledgers.

Problem Solved

As autonomous agents and MCP clients gain write and shell execution capabilities, treating raw LLM probability or tool-call intent as authorization introduces severe production outages and compliance blockers.

This server enforces a zero-trust execution boundary:

1. Never Equate Intent to Approval: High model confidence (model_confidence: 0.99) is explicitly rejected as authorization for destructive operations.
2. Tiered Tool Governance:
   • READ: Logged and allowed.
   • WRITE: Defaults to non-destructive simulation mode unless approved.
   • DESTRUCTIVE / PROVISION / DECOMMISSION: Hard DENY without a valid HITL prove token.
3. Immutable Action Ledger: Records an append-only JSONL ledger with SHA-256 hash-chain verification.
4. Atomic Kill-Switch: Instant freeze via environment variable (AAG_KILL_SWITCH=1) or file sentinel.

Testing & Validation

• Protocol: Fully JSON-RPC 2.0 stdio compliant.
• Test Suite: 21 automated unit and integration tests passing.
• Benchmarks: Includes cost avoidance models and ATT&CK evaluation cases (T1059, T1078, T1562, T1578).

Production & Commercial Context

Maintained and battle-tested by A2Z SOC for AI Forward Deployed Engineering, ISO 42001, NIST AI RMF, and SOC 2 Type II audit readiness.

For organizations deploying autonomous agent fleets requiring formal Gate/Prove audits or production hardening:

• Instant Audit ($499): https://a2zsoc.com/productized-services#instant-audit-tripwire
• Advisory & Sprints: https://a2zsoc.com/consultation

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