fix(demo): make validation suite runnable and shellcheck-clean

validate-all.sh aborted after the very first check because its
post-increment counters (`((VAR++))`) return exit status 1 when the old
value is 0, and `set -e` terminated the script. Use arithmetic
assignment so the suite runs to completion.

Add a .shellcheckrc to suppress the SC1090/SC1091 warnings that are
inherent to the environment-driven design (dynamic `source` of demo.env),
and drop an unused variable in demo-test.sh, so every script passes
shellcheck cleanly.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

demo/scripts/validate-all.sh

@@ -17,8 +17,8 @@ BLUE='\033[0;34m'
 NC='\033[0m'
 
 log_validation() { echo -e "${BLUE}[VALIDATE]${NC} $1"; }
-log_pass()       { echo -e "${GREEN}[PASS]${NC} $1"; ((VALIDATION_PASSED++)); }
-log_fail()       { echo -e "${RED}[FAIL]${NC} $1"; ((VALIDATION_FAILED++)); }
+log_pass()       { echo -e "${GREEN}[PASS]${NC} $1"; VALIDATION_PASSED=$((VALIDATION_PASSED + 1)); }
+log_fail()       { echo -e "${RED}[FAIL]${NC} $1"; VALIDATION_FAILED=$((VALIDATION_FAILED + 1)); }
 
 validate_yaml_files() {
     log_validation "Validating YAML files with yamllint..."