feat: add comprehensive architecture documentation

- Add AGENT-ONBOARDING.md for AI agents
- Add ARCHITECTURE.md with full system overview
- Add TECH-STACK.md with complete technology inventory
- Add DOMAIN-MODEL.md with entities and bounded contexts
- Add CODING-CONVENTIONS.md with patterns and practices
- Add GLOSSARY.md with terminology reference
- Add C4 diagrams (Context and Container levels)
- Add 10 ADRs documenting key decisions:
  - Talos Linux, NATS, MessagePack, Multi-GPU strategy
  - GitOps with Flux, KServe, Milvus, Dual workflow engines
  - Envoy Gateway
- Add specs directory with JetStream configuration
- Add diagrams for GPU allocation and data flows

Based on analysis of homelab-k8s2 and llm-workflows repositories
and kubectl cluster-info dump data.

diagrams/README.md

@@ -0,0 +1,35 @@
+# Diagrams
+
+This directory contains additional architecture diagrams beyond the main C4 diagrams.
+
+## Available Diagrams
+
+| File | Description |
+|------|-------------|
+| [gpu-allocation.mmd](gpu-allocation.mmd) | GPU workload distribution |
+| [data-flow-chat.mmd](data-flow-chat.mmd) | Chat request data flow |
+| [data-flow-voice.mmd](data-flow-voice.mmd) | Voice request data flow |
+
+## Rendering Diagrams
+
+### VS Code
+
+Install the "Markdown Preview Mermaid Support" extension.
+
+### CLI
+
+```bash
+# Using mmdc (Mermaid CLI)
+npx @mermaid-js/mermaid-cli mmdc -i diagram.mmd -o diagram.png
+```
+
+### Online
+
+Use [Mermaid Live Editor](https://mermaid.live)
+
+## Diagram Conventions
+
+1. Use `.mmd` extension for Mermaid diagrams
+2. Include title as comment at top of file
+3. Use consistent styling classes
+4. Keep diagrams focused (one concept per diagram)