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- Remove 9 outdated specification docs (v0.2.0 API design)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/reference/environment-variables.md

@@ -0,0 +1,32 @@
+# Environment Variables Reference
+
+Complete list of configuration variables.
+
+---
+
+See [docs/user-guide/configuration.md](../user-guide/configuration.md#environment-variables) for detailed descriptions.
+
+---
+
+## Required
+
+- `OPENAI_API_KEY`
+- `ALPHAADVANTAGE_API_KEY`
+- `JINA_API_KEY`
+
+---
+
+## Optional
+
+- `API_PORT` (default: 8080)
+- `API_HOST` (default: 0.0.0.0)
+- `OPENAI_API_BASE`
+- `MAX_CONCURRENT_JOBS` (default: 1)
+- `MAX_SIMULATION_DAYS` (default: 30)
+- `AUTO_DOWNLOAD_PRICE_DATA` (default: true)
+- `AGENT_MAX_STEP` (default: 30)
+- `VOLUME_PATH` (default: .)
+- `MATH_HTTP_PORT` (default: 8000)
+- `SEARCH_HTTP_PORT` (default: 8001)
+- `TRADE_HTTP_PORT` (default: 8002)
+- `GETPRICE_HTTP_PORT` (default: 8003)