docs: OpenAI ADK integration guide

[dgenio]

Context

Split from #78. A standalone integration guide for OpenAI's Agent Development Kit (ADK).

Proposal

Create docs/integration_openai_adk.md with:

• Architecture diagram showing contextweaver as a tool management layer for OpenAI ADK agents
• Code example demonstrating routing + firewall with OpenAI function calling
• Installation requirements (contextweaver + openai SDK)
• Common patterns: tool catalog management, context budget for function call arguments

Also create examples/openai_adk_demo.py with a runnable example.

Acceptance Criteria

• docs/integration_openai_adk.md with architecture diagram, code walkthrough, and gotchas
• examples/openai_adk_demo.py demonstrating routing + context compilation for OpenAI function calls
• README examples table updated
• Example runs without errors (mock LLM calls)

File Paths

• docs/integration_openai_adk.md (new)
• examples/openai_adk_demo.py (new)
• README.md (edit)

Parent Issue

Part of #78. Tracked by #123 (framework integration guides epic).