⚡ Allow targeted C++ lint preparation

[burgholzer]

🤖 AI text below 🤖

Description

C++ lint callers that only need generated headers currently have to build the entire default target. Add an optional build-target input so they can select their header-preparation target. An empty input preserves the current full build.

The target is passed through an environment variable and quoted shell array. Header-only callers can use the existing use-sccache-gha: false input because they do not compile C++ code.

Validation: uvx prek run -a passes, including actionlint and zizmor. The exact build-step script was exercised against a small CMake project: the empty input builds the default target, and a named target builds only that target.

GPT-6 via Codex implemented and validated this change at the maintainer's request.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

• Any agent that created, edited, or submitted GitHub content was explicitly authorized for that scope, as required by our AI Usage Guidelines.
• Every agent-authored or agent-edited public text body begins with the visible disclosure 🤖 *AI text below* 🤖 (titles are exempt).
• I have disclosed AI assistance in the PR description.
• I confirm that I have personally reviewed and understood all AI-generated content, and accept full responsibility for it.