feat: add conventional-commits commit-msg hook

README.md

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+# Client-Side Git Hooks
+
+To use one of these hooks, copy them from the respective directory to `.githooks/` in your repository and name according to the parent directory, then execute
+```shell
+git config core.hooksPath .githooks
+```
+
+It is recommended to then document when you made the copy so you can later propagate upstream changes, eg:
+```shell
+#!/bin/sh -euC
+#
+# Reject commit messages that do not conform to Conventional Commits.
+#
+# Enforces Conventional Commits v1.0.0 at the syntactic level.
+# Also enforces a custom list of types.
+#
+# Upstream:	gitea.bp99.eu/bp99/git-hooks
+# Updated:	2026-07-15
+
+# [... contents ...]
+```
+
+## Tricks
+
+To avoid having to remember the `git` command, add a `Makefile` to your project with a `setup` target:
+```makefile
+setup:
+	git config core.hooksPath .githooks
+
+.PHONY: setup
+```
+Do not forget to communicate to contributors that this `setup` target should be executed after cloning the repo.

commit-msg/conventional-commits-v1.0.0.commit-msg

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+#!/bin/sh -euC
+#
+# Reject commit messages that do not conform to Conventional Commits.
+#
+# Enforces Conventional Commits v1.0.0 at the syntactic level.
+#
+# Upstream:	gitea.bp99.eu/bp99/git-hooks
+
+subject="$(sed -n 1p "$1")"
+second="$(sed -n 2p "$1")"
+
+fail() {
+	>&2 cat <<EOF
+commit-msg: $1
+
+  subject: $subject
+  format: <type>[(<scope>)][!]: <description>
+
+Bypass with: git commit --no-verify
+EOF
+	exit 1
+}
+
+# type, optional (scope), optional !, terminal ': ', non-emtpy desc
+printf '%s' "$subject" | grep -Eq '^[[:alpha:]]+(\([^()]+\))?!?: .+' \
+|| fail 'subject does not conform to Conventional Commits'
+
+# Body must begin one blank line after description
+[ -z "$second" ] || fail 'Line 2 must be blank'