This commit adds a new GitHub Actions workflow that runs ShellCheck on all bash scripts
in the repository. ShellCheck is a static analysis tool for shell scripts that detects
bugs, security issues, and style problems.

Changes:
- Add .github/workflows/shellcheck.yml workflow that runs on push and pull requests
- Fix bash script issues in scripts/release-notes.sh:
  - Quote all variable expansions to prevent word splitting (SC2086)
  - Use proper error checking instead of checking $? (SC2181)

The workflow runs ShellCheck on all .sh files except testdata and generated directories.

Signed-off-by: Adesh Deshmukh <adeshkd123@gmail.com>