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>

README.md

@@ -186,6 +186,9 @@ zeroclaw channel bind-telegram 123456789
 # Get integration setup details
 zeroclaw integrations info Telegram
 
+# Note: Channels (Telegram, Discord, Slack) require daemon to be running
+# zeroclaw daemon
+
 # Manage background service
 zeroclaw service install
 zeroclaw service status
@@ -431,6 +434,12 @@ default_provider = "openrouter"
 default_model = "anthropic/claude-sonnet-4-20250514"
 default_temperature = 0.7
 
+# Custom OpenAI-compatible endpoint
+# default_provider = "custom:https://your-api.com"
+
+# Custom Anthropic-compatible endpoint
+# default_provider = "anthropic-custom:https://your-api.com"
+
 [memory]
 backend = "sqlite"              # "sqlite", "lucid", "markdown", "none"
 auto_save = true
@@ -531,6 +540,10 @@ api_url = "https://ollama.com"
 api_key = "ollama_api_key_here"
 ```
 
+### Custom Provider Endpoints
+
+For detailed configuration of custom OpenAI-compatible and Anthropic-compatible endpoints, see [docs/custom-providers.md](docs/custom-providers.md).
+
 ## Python Companion Package (`zeroclaw-tools`)
 
 For LLM providers with inconsistent native tool calling (e.g., GLM-5/Zhipu), ZeroClaw ships a Python companion package with **LangGraph-based tool calling** for guaranteed consistency: