docs: add AI Chat Showcase — public technical writeup for GitHub

doc/29-AI-CHAT-SHOWCASE.md: comprehensive technical showcase covering
the AI-powered CV navigation feature for potential clients/employers.

9 technical decisions explained with code:
1. ADK Go 1.0 as agent framework
2. Single agent, single tool design
3. Cross-section search across all CV data
4. CV navigation links (GPS for the CV)
5. Dual-provider with auto-fallback (Gemini→Ollama)
6. Model warmup on chat open
7. HTMX + plain JS (no SPA framework)
8. Rate limiting (30 req/hour/IP)
9. Graceful degradation

Linked from README.md and doc index.

README.md

@@ -256,6 +256,7 @@ This project includes comprehensive documentation organized by purpose:
 - **[ARCHITECTURE.md](doc/ARCHITECTURE.md)** - System design, patterns, and technical decisions
 - **[API.md](doc/API.md)** - Complete HTTP API reference and HTMX integration
 - **[AI-CHAT-AGENT.md](doc/28-AI-CHAT-AGENT.md)** - ADK Go agent architecture, tool design, and integration details
+- **[AI-CHAT-SHOWCASE.md](doc/29-AI-CHAT-SHOWCASE.md)** - Technical showcase: AI-powered CV navigation with ADK Go, dual-provider architecture, and document GPS
 
 ### 📋 Policies & Standards
 - **[SECURITY.md](doc/9-SECURITY.md)** - Complete security architecture, implementation, and testing guide