ci(github): configure dependabot automation

[ncipollina]

Summary

Updates this repo's Dependabot configuration to match the automation pattern used in sharp-mud. This adds grouped dependency update behavior, conventional Dependabot commit messages, GitHub Actions coverage, and automatic approval/auto-merge for safe Dependabot patch and minor updates.

Changes

• Expanded .github/dependabot.yml with cooldowns, conventional commit metadata, infrastructure multi-ecosystem grouping, and separate NuGet minor/patch vs. major groups.
• Added Dependabot checks for dotnet-sdk and github-actions ecosystems.
• Added .github/workflows/dependabot-auto-merge.yml to auto-approve and enable squash auto-merge for Dependabot semver patch/minor PRs only.
• Converted conditional Microsoft.Extensions.Logging.Abstractions package versions to bounded ranges so Dependabot can update lower bounds without crossing target-framework-specific major/preview lines.
• Added the new workflow file to LayeredCraft.StructuredLogging.slnx under /git/.

Validation

• Ran dotnet restore LayeredCraft.StructuredLogging.slnx successfully.
• Ran dotnet build LayeredCraft.StructuredLogging.slnx --no-restore successfully.
• Build completed with existing nullable warnings; no errors.

Notes for Reviewers

The net11.0 package range uses the same 12.0.0-a exclusive upper-bound pattern as sharp-mud to keep prerelease resolution below next-major previews.