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>

README.md

@@ -82,6 +82,7 @@ jobs:
 | `allowed_tools`       | Additional tools for Claude to use (the base GitHub tools will always be included)                                   | No       | ""        |
 | `disallowed_tools`    | Tools that Claude should never use                                                                                   | No       | ""        |
 | `custom_instructions` | Additional custom instructions to include in the prompt for Claude                                                   | No       | ""        |
+| `mcp_config`          | Additional MCP configuration (JSON string) that merges with the built-in GitHub MCP servers                          | No       | ""        |
 | `assignee_trigger`    | The assignee username that triggers the action (e.g. @claude). Only used for issue assignment                        | No       | -         |
 | `trigger_phrase`      | The trigger phrase to look for in comments, issue/PR bodies, and issue titles                                        | No       | `@claude` |
 
@@ -89,6 +90,61 @@ jobs:
 
 > **Note**: This action is currently in beta. Features and APIs may change as we continue to improve the integration.
 
+### Using Custom MCP Configuration
+
+The `mcp_config` input allows you to add custom MCP (Model Context Protocol) servers to extend Claude's capabilities. These servers merge with the built-in GitHub MCP servers.
+
+#### Basic Example: Adding a Sequential Thinking Server
+
+```yaml
+- uses: anthropics/claude-code-action@beta
+  with:
+    anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
+    mcp_config: |
+      {
+        "mcpServers": {
+          "sequential-thinking": {
+            "command": "npx",
+            "args": [
+              "-y",
+              "@modelcontextprotocol/server-sequential-thinking"
+            ]
+          }
+        }
+      }
+    allowed_tools: "mcp__sequential-thinking__sequentialthinking" # Important: Each MCP tool from your server must be listed here, comma-separated
+    # ... other inputs
+```
+
+#### Passing Secrets to MCP Servers
+
+For MCP servers that require sensitive information like API keys or tokens, use GitHub Secrets in the environment variables:
+
+```yaml
+- uses: anthropics/claude-code-action@beta
+  with:
+    anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
+    mcp_config: |
+      {
+        "mcpServers": {
+          "custom-api-server": {
+            "command": "npx",
+            "args": ["-y", "@example/api-server"],
+            "env": {
+              "API_KEY": "${{ secrets.CUSTOM_API_KEY }}",
+              "BASE_URL": "https://api.example.com"
+            }
+          }
+        }
+      }
+    # ... other inputs
+```
+
+**Important**:
+
+- Always use GitHub Secrets (`${{ secrets.SECRET_NAME }}`) for sensitive values like API keys, tokens, or passwords. Never hardcode secrets directly in the workflow file.
+- Your custom servers will override any built-in servers with the same name.
+
 ## Examples
 
 ### Ways to Tag @claude