Add opt-in shell lint and format targets

Introduce make lint-shell / fmt-shell with shellcheck and
shfmt (-i 2 -ci -bn), normalize first-party scripts, and
document the new targets. Keep them out of make lint / fmt.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

.shellcheckrc

@@ -0,0 +1,5 @@
+# Copyright (c) 2026 Probo Inc <hello@probo.com>.
+# SPDX-License-Identifier: MIT
+
+# Prefer shebang-detected dialect (sh vs bash).
+external-sources=false