tools(shellcheck): SHELLCHECK_INSTALL_ONLY env to skip lint passes

Allows CI workflows that need only the platform-detected ShellCheck binary
(reviewdog / action-suggester composers) to short-circuit after the cache
is populated, instead of running the full source/board/lib linting passes
they would have to discard.

Assisted-by: Claude:claude-opus-4.7
Signed-off-by: Igor Velkov <325961+iav@users.noreply.github.com>

Author: iav

lib/tools/shellcheck.sh

@@ -52,9 +52,9 @@ if [[ ! -f "${SHELLCHECK_BIN}" ]]; then
 	echo "Down URL: ${DOWN_URL}"
 	echo "SHELLCHECK_BIN: ${SHELLCHECK_BIN}"
 	curl -fLo "${SHELLCHECK_BIN}.tar.xz" "${DOWN_URL}" || {
-		echo "download of shellcheck failed from ${DOWN_URL}";
+		echo "download of shellcheck failed from ${DOWN_URL}"
 		rm -f "${SHELLCHECK_BIN}.tar.xz"
-		exit 1;
+		exit 1
 	}
 	tar -xf "${SHELLCHECK_BIN}.tar.xz" -C "${DIR_SHELLCHECK}" "shellcheck-v${SHELLCHECK_VERSION}/shellcheck"
 	mv -v "${DIR_SHELLCHECK}/shellcheck-v${SHELLCHECK_VERSION}/shellcheck" "${SHELLCHECK_BIN}"
@@ -63,6 +63,15 @@ if [[ ! -f "${SHELLCHECK_BIN}" ]]; then
 fi
 ACTUAL_VERSION="$("${SHELLCHECK_BIN}" --version | grep "^version")"
 
+# Install-only mode: callers that just need the binary (CI workflows
+# composing reviewdog / action-suggester on top of the framework's
+# platform-aware download cache) can short-circuit the linting passes
+# once SHELLCHECK_BIN is populated.
+if [[ -n "${SHELLCHECK_INSTALL_ONLY}" ]]; then
+	echo "SHELLCHECK_INSTALL_ONLY set, exiting after install. SHELLCHECK_BIN=${SHELLCHECK_BIN}"
+	exit 0
+fi
+
 function calculate_params_for_severity() {
 	declare SEVERITY="${SEVERITY:-"critical"}"