Add ShellCheck CI for scripts and embedded bash

## Summary
Introduce ShellCheck validation in CI for repository scripts and bash blocks maintained in workflows/action metadata where practical.

## Scope
- Add a CI job for `scripts/*.sh`
- Evaluate/check embedded bash snippets in action/workflow files with tooling or targeted lint strategy
- Fail CI on high-confidence shell correctness issues

## Why
Shell-heavy action logic now contains critical error-handling behavior; linting helps prevent subtle regressions.