feat: add conventional-commits commit-msg hook

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

@@ -0,0 +1,29 @@
+#!/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'