Migrate CI from Azure Pipelines to GitHub Actions

[lipkau]

Problem

The repository still relies on a legacy Azure DevOps pipeline (azure-pipelines.yml) using deprecated phase syntax and outdated hosted images. This creates drift from the rest of AtlassianPS and makes CI maintenance harder.

Proposal

Introduce a GitHub Actions CI workflow equivalent to the current build/test coverage, modeled after JiraPS conventions:

• separate lint/build/test jobs
• artifact handoff between jobs
• multi-OS test matrix (Windows PowerShell 5.1 + pwsh on Windows/Linux/macOS)
• required aggregate CI result job for branch protection

Acceptance criteria

• .github/workflows/ci.yml exists and runs on PRs and pushes to master
• Workflow runs lint/build/tests across target OS and shell matrix
• Test results artifacts are uploaded consistently
• Branch protection can rely on a single aggregate required check
• azure-pipelines.yml is removed or explicitly deprecated after parity confirmation