Fix assist_satellite config and validate app configs in CI

.github/scripts/run_shellcheck.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Run shellcheck over every shell script in the repository.
+#
+# The s6-overlay service scripts have no file extension and a
+# `#!/command/with-contenv bashio` shebang that shellcheck does not recognise,
+# so they are found by shebang and rely on their own `# shellcheck shell=bash`
+# directive.
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")/../.."
+
+declare -a scripts=()
+while IFS= read -r file; do
+    case "${file}" in
+        *.sh | *.bash)
+            scripts+=("${file}")
+            continue
+            ;;
+    esac
+    # Match an interpreter we can actually check, including the bashio wrapper
+    # used by the s6 run/finish scripts.
+    if IFS= read -r shebang <"${file}" 2>/dev/null &&
+        [[ "${shebang}" =~ ^#!.*(bash|bashio|/bin/sh|/usr/bin/env[[:space:]]+sh)([[:space:]]|$) ]]; then
+        scripts+=("${file}")
+    fi
+done < <(git ls-files)
+
+if [[ ${#scripts[@]} -eq 0 ]]; then
+    echo 'No shell scripts found' >&2
+    exit 1
+fi
+
+echo "Checking ${#scripts[@]} shell script(s):"
+printf '  %s\n' "${scripts[@]}"
+echo
+
+# -x lets shellcheck follow `source` directives between scripts.
+shellcheck --external-sources --color=always "${scripts[@]}"
+
+echo 'shellcheck found no issues'