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/.shellcheckrc

@@ -0,0 +1,5 @@
+# ShellCheck configuration for the TSYS Developer Support Stack
+# These rules are inherent to the environment-driven (dynamic source) design:
+#   SC1090 - can't follow non-constant source (e.g. `source "$ENV_FILE"`)
+#   SC1091 - not following external file specified as input (demo.env)
+disable=SC1090,SC1091