fix(imports): use clangd-friendly cpp imports

[ChEnylnti]

Summary

• replace the default C++ bits/stdc++.h import with standard library headers
• add IWYU pragma: keep to avoid clangd unused include diagnostics
• update the injector example in README

Motivation

This follows up on #216.

bits/stdc++.h is a GCC-specific header. On macOS, Apple clangd reports it as missing, which also breaks common completions and diagnostics for symbols like vector.

Since injected imports are editor-only and not submitted, using standard headers makes the default C++ editing experience more portable.

Tested locally with clangd: diagnostics are clean.