Docs: Add modernization skill documentation

Author: rvdginste

modernize_dotnet_build.md

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+# Skill: Modernize .NET Build & CI/CD
+
+## Objective
+Upgrade a legacy .NET project to use Central Package Management (CPM), standard build properties, locked dependencies, and comprehensive GitHub Actions pipelines.
+
+## Prerequisites
+*   Existing .NET solution.
+*   Access to reference configuration (e.g., `api-core`) for standard property files and rulesets.
+
+## Step-by-Step Procedure
+
+### 1. Clean & Standardize Configuration
+*   **NuGet:** Clean `NuGet.Config` (add `<clear />` to prevent pollution).
+*   **Git:** Update `.gitignore` to a modern standard (exclude IDE/editor noise).
+*   **Documentation:** Update `README.md` with standard project structure/links.
+*   **Legacy Removal:** Delete obsolete build scripts (e.g., `psake`, `build.bat`) and remove their references from the `.sln` file.
+
+### 2. Implement Modern Build Infrastructure
+*   **Properties:** Introduce `src/Directory.Build.props` (versioning, compiler flags) and `src/common.props` / `src/common-tests.props` (shared dependencies, rulesets).
+*   **Central Package Management:** Create `src/Directory.Packages.props`. Move all `PackageVersion` definitions from individual `.csproj` files to this central file.
+*   **Project Files:** Update `.csproj` files to remove explicit versions and import `common.props`.
+*   **Analyzers:** Rename/Update ruleset files (`StyleCop.ruleset` -> `Analyzer.ruleset`) to match the standard.
+
+### 3. Resolve Upgrade Issues
+*   **Code Fixes:** Run a build (`dotnet build`). Fix compilation errors caused by upgraded dependencies (e.g., moved exception types like `ApiUsageError`).
+*   **Static Analysis:** Address or suppress new StyleCop/Analyzer errors (e.g., `SA1402` split files, `SA16xx` documentation) to ensure a clean build. Use `Analyzer.ruleset` to suppress if necessary, or fix code structure.
+
+### 4. Finalize Solution Structure
+*   **Assets:** Update standard assets (e.g., logos) in the `assets/` folder.
+*   **Solution File:** Add the new configuration files (`.props`, `.ruleset`, `global.json`) to the Solution Items folder in `.sln` for easy access.
+
+### 5. Enforce Reproducibility
+*   **SDK Locking:** Create `global.json` to pin the .NET SDK version (e.g., to `10.0.102` or matching valid SDK).
+*   **Dependency Locking:** Run `dotnet restore --force-evaluate` to generate `packages.lock.json` files.
+
+### 6. Establish CI/CD Pipelines (GitHub Actions)
+*   **CI Pipeline:** Create `.github/workflows/ci.yml` for building and testing on every push/PR. Ensure it uses `global.json` and `--locked-mode`.
+*   **Publish Pipeline:** Create `.github/workflows/publish.yml` to package and push to NuGet/GitHub Releases on tag creation.
+*   **Reproducibility Workflow:** Create `.github/workflows/reproduce-build.yml` to verify that previous releases can be rebuilt bit-for-bit identical.