feat: add custom provider support to GitHub Action

[ArykLAnderson]

Summary

Ref: #1489

This PR adds comprehensive custom provider support to the opencode GitHub Action, enabling users to configure custom providers such as LiteLLM with secure environment variable injection.

Key Features Added

• Custom Config Files: Auto-discovery of .github/opencode.json (primary), opencode.json, and .opencode/config.json
• Environment Variable Injection: Support for config_env input
• Backward Compatibility: Existing workflows continue working unchanged
• Provider Integration: Proper integration with opencode's OPENCODE_CONFIG flag

New Action Inputs

Input	Description	Required	Default
model	Model to use (overrides config file)	No	-
opencode_config	Path to opencode config file	No	Auto-discovery
config_env	Environment variables for config (multiline key=value format)	No	-
share	Share the opencode session	No	true for public repos

Usage Examples

Basic custom provider:

- uses: sst/opencode/github@latest
  env:
    CUSTOM_API_KEY: ${{ secrets.CUSTOM_API_KEY }}
    BASE_URL=https://api.example.com
  # Automatically uses .github/opencode.json

   {
     "model": "custom-provider/my-model",
     "provider": {
       "custom-provider": {
         "npm": "@ai-sdk/openai-compatible",
         "models": {
           "my-model": {
             "name": "My Custom Model"
           }
         },
         "options": {
           "apiKey": "{env:CUSTOM_API_KEY}",
           "baseURL": "{env:CUSTOM_BASE_URL}"
         }
       }
     }
   }
   

Technical Implementation

• Environment Variable Parsing: Supports multiline key=value format for config_env
• Config Discovery: Prioritizes .github/opencode.json for GitHub-specific configurations
• Integration: Uses OPENCODE_CONFIG flag to properly integrate with opencode's config system

🤖 Generated with opencode