Document how to add a custom provider in README

[404-Page-Found]

Description

The AGENTS.md documents the provider system (create src/providers/my-provider.ts, add to BUILTIN_PROVIDERS, wire into createProvider()), but the user-facing README does not explain how to add a new provider or use the custom provider entry.

Problem Statement

Users who want to point commit-echo at a non-builtin OpenAI-compatible endpoint have no clear documentation, leading to confusion around the __custom__ provider key and CUSTOM_API_KEY env var.

Proposed Solution

Add a "Custom Providers" section to the README covering:

• How to use the custom provider key
• Setting COMMIT_ECHO_BASE_URL / CUSTOM_API_KEY
• How to contribute a new builtin provider

Additional Context

Relevant constants: CUSTOM_PROVIDER_KEY and CUSTOM_API_KEY_ENV in src/providers/registry.ts:3-4.

[truongsontung]

This is a great first issue! 📝 Documenting the custom provider flow would really help users who want to point commit-echo at their own OpenAI-compatible endpoints (local LLMs, proxies, etc.).

A few things that would be super helpful to document:

1. The __custom__ provider key — how to configure it via CUSTOM_API_KEY and CUSTOM_BASE_URL env vars
2. Example .env snippet — something users can copy-paste
3. Minimal provider implementation — a tiny my-provider.ts example showing the bare minimum (just chat.completions.create compatible)
4. Where to register it — pointer to BUILTIN_PROVIDERS / createProvider() in AGENTS.md

A quick-start code block like this would go a long way:

# .env
CUSTOM_API_KEY=sk-xxx
CUSTOM_BASE_URL=http://localhost:11434/v1

// src/providers/my-local.ts
export const myLocalProvider = createProvider({
  name: 'my-local',
  // ...minimal OpenAI-compatible implementation
});

Happy to help review a PR if someone picks this up! 🙌