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>

src/entrypoints/prepare.ts

@@ -84,11 +84,13 @@ async function run() {
     );
 
     // Step 11: Get MCP configuration
+    const additionalMcpConfig = process.env.MCP_CONFIG || "";
     const mcpConfig = await prepareMcpConfig(
       githubToken,
       context.repository.owner,
       context.repository.repo,
       branchInfo.currentBranch,
+      additionalMcpConfig,
     );
     core.setOutput("mcp_config", mcpConfig);
   } catch (error) {