docs: add custom provider endpoint configuration guide

Add comprehensive documentation for custom API endpoint configuration
to address missing documentation reported in issue #567.

Changes:
- Create docs/custom-providers.md with detailed guide for custom: and anthropic-custom: formats
- Add custom endpoint examples to README.md configuration section
- Add note about daemon requirement for channels in Quick Start
- Add reference link to custom providers guide

Addresses: #567

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/custom-providers.md

@@ -0,0 +1,99 @@
+# Custom Provider Configuration
+
+ZeroClaw supports custom API endpoints for both OpenAI-compatible and Anthropic-compatible providers.
+
+## Provider Types
+
+### OpenAI-Compatible Endpoints (`custom:`)
+
+For services that implement the OpenAI API format:
+
+```toml
+default_provider = "custom:https://your-api.com"
+api_key = "your-api-key"
+default_model = "your-model-name"
+```
+
+### Anthropic-Compatible Endpoints (`anthropic-custom:`)
+
+For services that implement the Anthropic API format:
+
+```toml
+default_provider = "anthropic-custom:https://your-api.com"
+api_key = "your-api-key"
+default_model = "your-model-name"
+```
+
+## Configuration Methods
+
+### Config File
+
+Edit `~/.zeroclaw/config.toml`:
+
+```toml
+api_key = "your-api-key"
+default_provider = "anthropic-custom:https://api.example.com"
+default_model = "claude-sonnet-4"
+```
+
+### Environment Variables
+
+```bash
+export ANTHROPIC_API_KEY="your-api-key"
+zeroclaw agent
+```
+
+## Testing Configuration
+
+Verify your custom endpoint:
+
+```bash
+# Interactive mode
+zeroclaw agent
+
+# Single message test
+zeroclaw agent -m "test message"
+```
+
+## Troubleshooting
+
+### Authentication Errors
+
+- Verify API key is correct
+- Check endpoint URL format (must include `https://`)
+- Ensure endpoint is accessible from your network
+
+### Model Not Found
+
+- Confirm model name matches provider's available models
+- Check provider documentation for exact model identifiers
+
+### Connection Issues
+
+- Test endpoint accessibility: `curl -I https://your-api.com`
+- Verify firewall/proxy settings
+- Check provider status page
+
+## Examples
+
+### Local LLM Server
+
+```toml
+default_provider = "custom:http://localhost:8080"
+default_model = "local-model"
+```
+
+### Corporate Proxy
+
+```toml
+default_provider = "anthropic-custom:https://llm-proxy.corp.example.com"
+api_key = "internal-token"
+```
+
+### Cloud Provider Gateway
+
+```toml
+default_provider = "custom:https://gateway.cloud-provider.com/v1"
+api_key = "gateway-api-key"
+default_model = "gpt-4"
+```