fix(mcp): restrict stdio server environment

[liuedcson]

Summary

• inherit only the MCP SDK safe default environment for stdio servers
• preserve explicitly declared server variables and the Step login credential fallback
• update plugin diagnostics so they no longer suggest exporting variables that will not be inherited
• add regression coverage proving unrelated parent-process secrets are excluded

Testing

• pnpm --filter @step-harness/coding-agent exec vitest --run src/step/mcp.test.ts test/step-plugins.test.ts
• pnpm --filter @step-harness/coding-agent test
• pnpm run check