Add OCI enterprise AI skill

[RahulMR42]

• ## 🚀 Summary

• Added a new OCI Enterprise AI skill under oci/enterprise-ai/.
• Positioned Enterprise AI as a subset of the broader OCI skill domain.
• Updated the OCI root skill to route users into Enterprise AI workflows.
• Updated repository navigation and authoring guidance for nested skills.

✨ What’s Included

• 🤖 OCI Generative AI model selection and endpoint guidance
• 🧠 Responses API agent workflow guidance
• 🔎 RAG, File Search, Vector Stores, SQL Search, and retrieval routing
• 🛠️ Agent tool guidance for Code Interpreter, Function Calling, MCP, and xAI-compatible tools
• 🔐 Governance guidance for IAM, API keys, private endpoints, OAuth, guardrails, and ZPR
• 📦 Hosted agentic application guidance
• 💸 OCI Generative AI and Generative AI Agents cost-estimation guidance
• 🧩 Custom, fine-tuned, and imported model lifecycle guidance
• 🔗 Oracle platform integration routing for Autonomous Database, APEX, and OCI AI Services

🧭 Structure Changes

• Added oci/enterprise-ai/ as a nested OCI skill.
• Expanded oci/SKILL.md from a placeholder into an OCI domain router.
• Updated README.md to show Enterprise AI under OCI.
• Updated SKILL_AUTHORING_GUIDE.md to allow nested skills for large domain capabilities.

[krisrice]

Held during review. GitHub reported this PR as DIRTY earlier, and a local merge-tree against current main showed conflicts in oci/SKILL.md. Please rebase or merge main and resolve the OCI domain index conflict before merge.

[RahulMR42]

@krisrice - Fixed the conflicts & thanks in advance for your review.