Add ShellCheck and unify CI workflows

[yubiuser]

What does this PR aim to accomplish?:

Adds the same shellcheck action as we have it in pihole/pihole and puts all CI workflows in a single file

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[Copilot]

Pull Request Overview

This PR adds ShellCheck static analysis and consolidates all CI workflows into a single file for better maintainability. The changes address ShellCheck warnings by improving shell script best practices throughout the codebase.

• Adds ShellCheck configuration and integrates it into CI workflows
• Consolidates separate CI workflow files into a unified CI.yml file
• Updates shell script to follow ShellCheck recommendations for variable quoting and comparison operators

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
padd.sh	Updated variable quoting and comparison operators to follow ShellCheck best practices
.shellcheckrc	Added ShellCheck configuration with custom rules and disabled checks
.github/workflows/CI.yml	New unified CI workflow combining shellcheck, codespell, and editorconfig checks
.github/workflows/editorconfig-checker.yml	Removed standalone editorconfig workflow (consolidated into CI.yml)
.github/workflows/codespell.yml	Removed standalone codespell workflow (consolidated into CI.yml)

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Conflicts have been resolved.