feat: Add Custom Anthropic Model Option Support

[ibrahimsn98]

Add Configurable Claude Model via Environment Variable

Problem

The Claude model was hardcoded to claude-sonnet-4-5-20250929 in src/ai/claude-executor.ts. Users with custom Anthropic-compatible endpoints
(proxies, API gateways, or alternative deployments) had no way to specify a different model name, even when their endpoint required it.

Solution

Introduce the ANTHROPIC_MODEL environment variable that allows users to override the default model. This works seamlessly with the existing
ANTHROPIC_BASE_URL support, enabling full customization of both the endpoint and the model.

Changes

File	Change
src/ai/claude-executor.ts	Use process.env.ANTHROPIC_MODEL with fallback to default model
docker-compose.yml	Pass ANTHROPIC_MODEL environment variable to worker container
.env.example	Add "Option 3" documentation for custom endpoint + model configuration
README.md	Add "Custom Anthropic Endpoint" section with usage instructions

Usage

# In .env
ANTHROPIC_API_KEY=your-api-key
ANTHROPIC_BASE_URL=https://your-custom-endpoint.com  # Optional
ANTHROPIC_MODEL=claude-sonnet-4-5-20250929              # Optional

# Run
./shannon start URL=https://example.com REPO=/path/to/repo

If ANTHROPIC_MODEL is not set, the default claude-sonnet-4-5-20250929 is used.