Add Unity AI Gateway governance demo

[dmatrix]

What

Adds a self-contained demo — demos/unity_ai_gateway_governance/ — showing how to govern coding-agent sprawl (Cursor, Claude Code, Codex CLI, Gemini CLI, Pi) through Unity AI Gateway. Each provider is routed to its own governed model service (catalog.schema.service) carrying guardrails, an inference-table audit trail, usage tracking, and rate limits, all reached through one gateway URL.

The notebook walks through:

• Act 1 — verify each governed model service and its deployed config
• Act 2 — simulate the coding-agent swarm (happy path)
• Act 3 — guardrails in action (PII, jailbreak, unsafe content)
• Acts 4–5 — audit trail and usage tracking via inference tables + system.ai_gateway.usage
• Act 6 — per-service QPM/TPM rate limiting
• Act 7–8 — MLflow tracing and a summary dashboard

Changes

• New demo project under demos/unity_ai_gateway_governance/ (notebook + supporting modules, images, DAB config, env template)
• Link the demo from the top-level README.md
• Default Act 2 happy-path volume to 1 clean request per agent, interleaved and paced, to stay under shared foundation-model backend and guardrail-judge per-minute limits (configurable via get_clean_scenarios(per_agent=...))
• Add @dmatrix to CODEOWNERS.txt

Secrets are excluded via .gitignore (.env, .databricks/, mlartifacts/); env-template ships with placeholders only.

This pull request and its description were written by Isaac.