Repo health: modernize deps, security, tests, docs

[hippi345]

Summary

Modernizes the AzCopyAndDelete C# console app from .NET Framework 4.6.1 with committed NuGet packages to a .NET 8 SDK-style solution with current Azure SDKs, offline unit tests, CI, and documentation.

Security

• No committed secrets found in source history reviewed for this pass; credentials were only ever collected at runtime via console prompts.
• Removed vendored packages/, bin/, and obj/ artifacts from version control.
• Added .gitignore, SECURITY.md, and documented configuration via environment variables (no real secrets in repo).
• Replaced deprecated WindowsAzure.Storage / Microsoft.WindowsAzure.Storage with Azure.Storage.Blobs 12.x.

Dependencies and runtime

• Upgraded target framework from .NET Framework 4.6.1 to .NET 8.
• Switched to SDK-style projects with NuGet restore (no packages.config).
• Removed unused Microsoft.Azure.Storage.DataMovement dependency.
• Added global.json and Directory.Build.props for consistent SDK/analyzer settings.

Working code

• Preserved interactive backup flow: upload file to blob container, optional blob listing, optional local delete.
• Added non-interactive mode via AZURE_* environment variables.
• Validates source path exists before upload.
• Project layout moved to src/AzCopyAndDelete/ with solution at repo root.

Tests and linting

• Added 16 xUnit tests (mocked blob/local services; no Azure credentials required).
• dotnet format enforced in CI; .editorconfig added.

CI

• GitHub Actions workflow: restore, format check, build (Release), test on push and pull_request.

License

• Added MIT License (copyright 2026 Joel Shearon); no prior LICENSE file existed.

README

• New README with features, requirements, env var configuration, usage, test instructions, and project structure; CI and license badges.

General polish

• .editorconfig, Dependabot for nuget and github-actions, removed dead build artifacts.

Verification

• Local: dotnet build, dotnet test, dotnet format --verify-no-changes — all passing.

Owner actions

• None required for leaked secrets from this review.
• Install .NET 8 SDK locally to build; open solution AzCopyAndDelete.sln at repo root (old AzCopyAndDelete/AzCopyAndDelete.sln path removed).