Fix MCP server undefined error and file path resolution

- Add error field to MCP error responses to fix 'undefined' errors
- Add REPO_DIR environment variable to fix file path resolution
- Use GITHUB_WORKSPACE for correct repository directory
- Simplify path processing logic in commit_files tool

This fixes the issue where mcp__github_file_ops__commit_files would fail
with 'Error calling tool commit_files: undefined' by ensuring error messages
are properly formatted and files are read from the correct directory.

src/mcp/install-mcp-server.ts

@@ -34,6 +34,7 @@ export async function prepareMcpConfig(
             REPO_OWNER: owner,
             REPO_NAME: repo,
             BRANCH_NAME: branch,
+            REPO_DIR: process.env.GITHUB_WORKSPACE || process.cwd(),  // Use GitHub workspace directory
           },
         },
       },