docs: add CLAUDE.md

CLAUDE.md

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+# CLAUDE.md
+
+## Style
+
+Keep everything short — replies, explanations, comments, docs.
+
+## Commits
+
+- English only.
+- Conventional Commits prefix: `feat:`, `fix:`, `chore:`, `docs:`, `refactor:`, `ci:`, `build:`.
+- Subject as short as possible. Imperative, lowercase, no trailing period.
+- Body only when something genuinely cannot be inferred from the diff.
+- Exception: the commits in `build/upstream` that `patches/` is exported from. Their subjects become the patch file names, so they stay plain and descriptive.