Add CI workflows for testing and version checks, enhance README and T…

[locus313]

This pull request introduces a comprehensive continuous integration (CI) setup and significantly improves the testing infrastructure for the ssh-key-sync project. It adds new GitHub Actions workflows for CI, testing, and linting, expands documentation with a dedicated testing guide, and enhances script robustness and portability. The most important changes are grouped below:

CI/CD and Testing Infrastructure

• Added a main CI workflow (.github/workflows/ci.yml) that orchestrates linting, testing, and version checks, ensuring all checks are run on pull requests and pushes to main ([.github/workflows/ci.ymlR1-R46]).
• Introduced a detailed test workflow (.github/workflows/test.yml) with both unit and integration tests for the sync-ssh-keys.sh script, including error handling, configuration validation, and permission checks ([.github/workflows/test.ymlR1-R284]).
• Updated the lint workflow to trigger on pushes and pulls for relevant files, improving code quality enforcement ([.github/workflows/lint.ymlR9-R16]).
• Improved the version check workflow to monitor its own file for changes ([.github/workflows/check-version.ymlR10]).

Documentation and Testing Guide

• Expanded the README.md with a new "Testing" section, CI status badge, and instructions for running tests locally [1]], [2]]).
• Added a comprehensive TESTING.md guide describing CI/CD, local testing, test coverage, and contribution guidelines ([TESTING.mdR1-R221]).

Script Improvements and Portability

• Enhanced the version update logic in sync-ssh-keys.sh to version 0.1.3 ([sync-ssh-keys.shL10-R10]).
• Made the authorized_keys update logic more portable by supporting cmp, diff, and checksums for file comparison, improving compatibility across environments ([sync-ssh-keys.shL325-R356]).
• Improved temporary file handling and variable scoping in the main function for better reliability and cleanup [1]], [2]]).This pull request introduces a comprehensive automated testing framework for the ssh-key-sync project, including new CI workflows, unit and integration tests, and updated documentation. The changes ensure the script is robust, well-tested, and that all contributions are validated automatically via GitHub Actions. The most important changes are grouped below.

Continuous Integration and Workflow Automation:

• Added a main CI workflow (.github/workflows/ci.yml) that orchestrates linting, testing, and version checks on all pull requests and pushes to main, with a final status check to ensure all jobs pass.
• Introduced a dedicated test workflow (.github/workflows/test.yml) that performs extensive unit and integration tests, including configuration validation, error handling, mock network tests, and real user/SSH key synchronization in a safe environment.
• Improved the lint and version check workflows to trigger on relevant file changes and both push and pull request events, ensuring up-to-date validation. [1] [2]

Testing Infrastructure and Scripts:

• Added a standalone unit test script (test.sh) for local validation, checking script existence, syntax, function presence, argument handling, and configuration loading.

Documentation Updates:

• Updated README.md with a new Testing section, CI status badge, and instructions for running tests locally. [1] [2]
• Added a comprehensive TESTING.md guide detailing the CI setup, test coverage, local testing procedures, and best practices for contributing new tests.

These changes significantly improve the project's reliability and make it easier for contributors to ensure their changes are well-tested and compliant with project standards.…ESTING documentation