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/gpu-allocation.mmd

@@ -0,0 +1,47 @@
+%% GPU Allocation Diagram
+%% Shows how AI workloads are distributed across GPU nodes
+
+flowchart TB
+    subgraph khelben["🖥️ khelben (AMD Strix Halo 64GB)"]
+        direction TB
+        vllm["🧠 vLLM<br/>LLM Inference<br/>100% GPU"]
+    end
+
+    subgraph elminster["🖥️ elminster (NVIDIA RTX 2070 8GB)"]
+        direction TB
+        whisper["🎤 Whisper<br/>STT<br/>~50% GPU"]
+        xtts["🔊 XTTS<br/>TTS<br/>~50% GPU"]
+    end
+
+    subgraph drizzt["🖥️ drizzt (AMD Radeon 680M 12GB)"]
+        direction TB
+        embeddings["📊 BGE Embeddings<br/>Vector Encoding<br/>~80% GPU"]
+    end
+
+    subgraph danilo["🖥️ danilo (Intel Arc)"]
+        direction TB
+        reranker["📋 BGE Reranker<br/>Document Ranking<br/>~80% GPU"]
+    end
+
+    subgraph workloads["Workload Routing"]
+        chat["💬 Chat Request"]
+        voice["🎤 Voice Request"]
+    end
+
+    chat --> embeddings
+    chat --> reranker
+    chat --> vllm
+
+    voice --> whisper
+    voice --> embeddings
+    voice --> reranker
+    voice --> vllm
+    voice --> xtts
+
+    classDef nvidia fill:#76B900,color:white
+    classDef amd fill:#ED1C24,color:white
+    classDef intel fill:#0071C5,color:white
+    
+    class whisper,xtts nvidia
+    class vllm,embeddings amd
+    class reranker intel