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>

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 }}