docs: add guide on setting up a clang-format pre-commit hook

[shenxianpeng]

Why

cpp-linter-hooks already gets more Google traffic than the action (52 of its 288 repository views in the last 14 days came from Google, against 21 for cpp-linter-action), so people are searching for "clang-format pre-commit hook". The site has no page for that query. This is the second how-to written around a search term, after #62; the two PRs are independent.

What's in this PR

One new post in the Guides category: Set up a clang-format pre-commit hook for C and C++ (docs/blog/posts/2026-09-20-clang-format-pre-commit-hook.md), published at /blog/2026/09/20/clang-format-pre-commit-hook/ (explicit slug, plus a meta description).

1. A starter .clang-format.
2. Installing pre-commit and adding the hook; rev is the hook version, --version is the clang-format version (21 resolves to the newest 21.x wheel, a full version pins it exactly).
3. What the first commit looks like, and why "Failed" means the files were fixed.
4. Formatting only your own code: exclude, a DisableFormat .clang-format in the vendored directory, and types_or for CUDA and Protobuf.
5. Existing code bases: reformat once with .git-blame-ignore-revs, or format as you go; git clang-format mentioned for changed-lines-only.
6. Enforcing the same configuration in CI with pre-commit run --all-files --show-diff-on-failure, or review suggestions from cpp-linter-action with format-review.
7. A troubleshooting table.
8. When to add the clang-tidy hook.

Checks

• Every command and output in the post was run in a scratch git repository with rev: v1.6.0 and --version=21 (resolved to clang-format 21.1.8): the blocked commit and the retry, exclude, the per-directory DisableFormat, .cu and .proto formatting via types_or, SKIP=clang-format, the CI output (pasted verbatim) and blame.ignoreRevsFile.
• The compile database auto-detection mentioned in the last section was checked against clang_tidy.py at v1.6.0.
• mkdocs build --strict passes locally, and the repository's pre-commit hooks pass on the new file.

Notes

• The post does not recommend --dry-run. With the released v1.6.0 the hook reports "Passed" for an unformatted file when --dry-run is set, with or without --Werror; the fix (fix: make --dry-run correctly report formatting issues cpp-linter-hooks#257) is on main but not in a release yet. Once it is released, a check-only variant can be added to step 6.
• There is no link to the clang-tidy guide from docs: add guide on running clang-tidy on pull requests with GitHub Actions #62, because that page does not exist on main yet and the strict build would fail. The two posts can be cross-linked in a small follow-up once both are merged.

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