Add databricks-ai-gateway skill

[Praneeth16]

Summary

• Adds a new databricks-ai-gateway skill covering both V1 (per-endpoint config) and V2 Beta (enterprise control plane)
• V1 coverage: guardrails (PII, safety, keywords, topics, LLM judge, custom), rate limits (QPM/TPM), inference tables, fallbacks, traffic splitting, observability via system tables
• V2 coverage: coding agent governance (Cursor, Codex CLI, Gemini CLI, Claude Code), MCP server access control, agentic multi-step traceability, unified dashboard
• Registers skill in install_skills.sh and adds cross-references to databricks-model-serving and databricks-agent-bricks

Files

File	Purpose
SKILL.md	Main entrypoint with V1/V2 decision tables, quick starts, SDK reference, common issues
1-guardrails.md	PII BLOCK/MASK/NONE, safety, keywords, topics, LLM judge, custom guardrails
2-rate-limits-and-fallbacks.md	QPM/TPM hierarchy, fallback config, traffic splitting patterns
3-inference-tables.md	Payload logging to UC Delta tables
4-observability.md	System tables, cost attribution, audit trail queries
5-coding-agents.md	Cursor, Codex CLI, Gemini CLI, Claude Code gateway routing setup
6-agentic-governance.md	MCP governance, multi-step traceability, unified control plane

Test plan

• install_skills.sh --list shows the new skill
• All internal links (6 sub-files) resolve correctly
• All cross-reference links (5 related skills) resolve correctly
• No sub-files have accidental frontmatter
• All 10 SDK class names verified against actual SDK source
• Rate limits + usage tracking tested on custom model endpoint (sn-lead-scoring-praneeth_paikray on e2-demo-west)
• Full config (guardrails + rate limits + usage tracking) tested on external model endpoint (costco-meat-ai-gateway on e2-demo-west)
• Guardrails correctly rejected on custom model endpoint (documented limitation)
• Both endpoints cleaned up and restored to original state

This pull request was AI-assisted by Isaac.

[dustinvannoy-db]

I like the idea of a skill for this, would like to try and reduce size and also use references/ folder for the extra markdowns. @jacksandom do you have any time to guide Praneeth on setting up evals and trying to reduce size?