docs: Google ADK (Vertex AI) integration guide

[dgenio]

Context

Split from #78. A standalone integration guide for Google's Agent Development Kit (ADK / Vertex AI agents).

Proposal

Create docs/integration_google_adk.md with:

• Architecture diagram showing contextweaver as a context layer for Google ADK agents
• Code example demonstrating routing + firewall with Google's tool calling API
• Installation requirements (contextweaver + google-cloud-aiplatform or google-genai)
• Common patterns: catalog management for Google tools, phase-based context for structured outputs

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

Acceptance Criteria

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

File Paths

• docs/integration_google_adk.md (new)
• examples/google_adk_demo.py (new)
• README.md (edit)

Parent Issue

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