docs: restructure documentation for improved clarity and navigation

Reorganize documentation into user-focused, developer-focused, and deployment-focused sections.

**New structure:**
- Root: README.md (streamlined), QUICK_START.md, API_REFERENCE.md
- docs/user-guide/: configuration, API usage, integrations, troubleshooting
- docs/developer/: contributing, development setup, testing, architecture
- docs/deployment/: Docker deployment, production checklist, monitoring
- docs/reference/: environment variables, MCP tools, data formats

**Changes:**
- Streamline README.md from 831 to 469 lines
- Create QUICK_START.md for 5-minute onboarding
- Create API_REFERENCE.md as single source of truth for API
- Remove 9 outdated specification docs (v0.2.0 API design)
- Remove DOCKER_API.md (content consolidated into new structure)
- Remove docs/plans/ directory with old design documents
- Update CLAUDE.md with documentation structure guide
- Remove orchestration-specific references

**Benefits:**
- Clear entry points for different audiences
- No content duplication
- Better discoverability through logical hierarchy
- All content reflects current v0.3.0 API

docs/developer/adding-models.md

@@ -0,0 +1,69 @@
+# Adding Custom AI Models
+
+How to add and configure custom AI models.
+
+---
+
+## Basic Setup
+
+Edit `configs/default_config.json`:
+
+```json
+{
+  "models": [
+    {
+      "name": "Your Model Name",
+      "basemodel": "provider/model-id",
+      "signature": "unique-identifier",
+      "enabled": true
+    }
+  ]
+}
+```
+
+---
+
+## Examples
+
+### OpenAI Models
+
+```json
+{
+  "name": "GPT-4",
+  "basemodel": "openai/gpt-4",
+  "signature": "gpt-4",
+  "enabled": true
+}
+```
+
+### Anthropic Claude
+
+```json
+{
+  "name": "Claude 3.7 Sonnet",
+  "basemodel": "anthropic/claude-3.7-sonnet",
+  "signature": "claude-3.7-sonnet",
+  "enabled": true,
+  "openai_base_url": "https://api.anthropic.com/v1",
+  "openai_api_key": "your-anthropic-key"
+}
+```
+
+### Via OpenRouter
+
+```json
+{
+  "name": "DeepSeek",
+  "basemodel": "deepseek/deepseek-chat",
+  "signature": "deepseek",
+  "enabled": true,
+  "openai_base_url": "https://openrouter.ai/api/v1",
+  "openai_api_key": "your-openrouter-key"
+}
+```
+
+---
+
+## Field Reference
+
+See [docs/user-guide/configuration.md](../user-guide/configuration.md#model-configuration-fields) for complete field descriptions.