ci: enable Dependabot (monthly, grouped) and CodeQL

[lippdev]

Summary

• Dependabot: NuGet and GitHub Actions, monthly, each ecosystem grouped into a single PR. CI validates each one on Windows.
• CodeQL: C# (no build needed) and workflow analysis on PRs, on main and weekly. Free for public repos; findings go to the Security tab.

🤖 Generated with Claude Code

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.

[pullfrog]

Important

The NuGet Dependabot entry points at a directory with no NuGet manifest, so the configured .NET dependencies will not receive version updates.

Reviewed changes This review covers the new Dependabot configuration and CodeQL workflow.

• Dependabot — Adds monthly grouped updates for NuGet and GitHub Actions.
• CodeQL — Adds C# and GitHub Actions analysis for pull requests, pushes to main, and a weekly schedule.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using GPT Luna ｜ 𝕏}

[pullfrog]

directory: "/" points NuGet discovery at the repository root, but both NuGet manifests are nested under src/ConsoleMode and tests/ConsoleMode.Tests. Dependabot will therefore find no project to update for this entry; configure both manifest directories (or use the directories option) so the app and test dependencies are covered.

Technical details

# NuGet manifests are outside the configured directory

## Affected sites
- `.github/dependabot.yml:6` — configures the NuGet scan at the repository root.
- `src/ConsoleMode/ConsoleMode.csproj:49-54` — contains the app's NuGet references.
- `tests/ConsoleMode.Tests/ConsoleMode.Tests.csproj:13-16` — contains the test project's NuGet references.

## Required outcome
- Configure NuGet discovery for both `/src/ConsoleMode` and `/tests/ConsoleMode.Tests`, or an equivalent supported multi-directory configuration.

## Suggested approach
- Replace the single root entry with two NuGet entries, or use `directories` if the repository's Dependabot configuration version supports it.

## Sources
- https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#directory
- https://docs.github.com/en/code-security/reference/supply-chain-security/supported-ecosystems-and-manifests-for-dependency-scope