feat: add Exogram Authority Runtime reference server

[Richard-Ewing]

This PR adds the Exogram MCP reference server.

Exogram provides the Authority Runtime and Cognitive Filter for AI agents. This server exposes tools to:

1. Evaluate & Commit Actions: Intercept tool calls, cryptographically validate payload admissibility against strict enterprise policies, and commit the execution to an immutable audit ledger.
2. Store & Search Knowledge: Read and write persistent context to the Exogram Cognitive Filter (a SQL-backed knowledge graph with vector semantics) to eliminate the 'Lost in the Middle' syndrome and prevent context poisoning.