feat: add mcp_config input that merges with existing mcp server (#96)

* feat: add mcp_config input that merges with existing mcp server

- Add mcp_config input parameter to action.yml 
- Modify prepareMcpConfig() to accept and merge additional config
- Provided config overrides built-in servers in case of naming collisions
- Pass MCP_CONFIG environment variable from action to prepare step

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* refactor: improve MCP config validation and merging logic

- Add JSON validation to ensure parsed config is an object
- Simplify merge logic with explicit mcpServers merging
- Enhance error logging with config preview for debugging

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* refactor: improve MCP config logging per review feedback

- Remove configPreview from error logging to avoid cluttering output
- Add informational log when merging MCP server configurations
- Simplify error message for failed config parsing

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* test: add comprehensive unit tests for prepareMcpConfig

Add tests covering:
- Basic functionality with no additional config
- Valid JSON merging scenarios
- Invalid JSON handling
- Empty/null config handling
- Server name collision scenarios
- Complex nested configurations
- Environment variable handling

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* docs: add mcp_config example with sequential-thinking server

- Add mcp_config to inputs table
- Add example section showing how to use mcp_config with sequential-thinking MCP server
- Include clear explanation that custom servers override built-in servers

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* readme

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

action.yml

@@ -39,6 +39,10 @@ inputs:
     description: "Direct instruction for Claude (bypasses normal trigger detection)"
     required: false
     default: ""
+  mcp_config:
+    description: "Additional MCP configuration (JSON string) that merges with the built-in GitHub MCP servers"
+    required: false
+    default: ""
 
   # Auth configuration
   anthropic_api_key:
@@ -92,6 +96,7 @@ runs:
         ALLOWED_TOOLS: ${{ inputs.allowed_tools }}
         CUSTOM_INSTRUCTIONS: ${{ inputs.custom_instructions }}
         DIRECT_PROMPT: ${{ inputs.direct_prompt }}
+        MCP_CONFIG: ${{ inputs.mcp_config }}
         OVERRIDE_GITHUB_TOKEN: ${{ inputs.github_token }}
         GITHUB_RUN_ID: ${{ github.run_id }}