docs: Add comprehensive deployment and variables reference

Add detailed documentation to enable autonomous deployment by other AI agents.

New files:
- DEPLOYMENT.md: Complete step-by-step deployment guide
  - Prerequisites checklist
  - Required vs optional variables clearly marked
  - All configuration variables documented with examples
  - Troubleshooting section
  - Quick deploy example with all values
  - Configuration summary table

- VARIABLES.md: Complete variables reference
  - All variables listed with file, line, type, description
  - Required variables clearly marked
  - Optional variables with defaults
  - Variables grouped by use case
  - Quick copy templates
  - Environment variables reference

Updated README.md:
- Add links to new documentation files
- Clear pointer to DEPLOYMENT.md for deployment

These docs enable another Claude Code instance to:
1. Identify all required configuration variables
2. Know where to find/set each variable
3. Understand what values are valid
4. Deploy successfully without human intervention

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

README.md

@@ -35,8 +35,16 @@ A containerized development environment with GUI access, featuring:
 - Workspace mounted at `/workspace`
 - Repository cloned on first startup
 
+## Documentation
+
+- **[DEPLOYMENT.md](DEPLOYMENT.md)** - Complete deployment guide with step-by-step instructions
+- **[VARIABLES.md](VARIABLES.md)** - Reference for all configuration variables
+- **[README.md](README.md)** - This file (overview and quick start)
+
 ## Quick Start
 
+**👉 For detailed deployment instructions, see [DEPLOYMENT.md](DEPLOYMENT.md)**
+
 ### 1. Get the Image
 
 The image is automatically built and published to GitHub Container Registry on every push to main.