chore: Layer 3 polish — SHA pinning, modern C# rules, metadata, trim/AOT

- Pin all GitHub Actions to full commit SHA with version comments
- Add modern C# editorconfig rules (primary constructors, pattern matching,
  collection expressions, readonly structs, UTF-8 literals)
- Add Copyright and Company NuGet metadata in Directory.Build.props
- Enable IsTrimmable and IsAotCompatible for packable projects
- Add CONTRIBUTING.md with build commands and development workflow
- Enhance PR template with quality gate checklist

Made-with: Cursor

Author: Hongwei-MB

.editorconfig

@@ -25,6 +25,17 @@ csharp_prefer_braces = true:suggestion
 csharp_prefer_simple_using_statement = true:suggestion
 csharp_style_prefer_switch_expression = true:suggestion
 csharp_style_prefer_pattern_matching = true:suggestion
+csharp_style_prefer_not_pattern = true:suggestion
+csharp_style_prefer_extended_property_pattern = true:suggestion
+csharp_style_prefer_primary_constructors = true:suggestion
+csharp_style_prefer_top_level_statements = true:silent
+csharp_style_prefer_method_group_conversion = true:suggestion
+csharp_style_prefer_null_check_over_type_check = true:suggestion
+csharp_style_prefer_tuple_swap = true:suggestion
+csharp_style_prefer_utf8_string_literals = true:suggestion
+csharp_style_implicit_object_creation_when_type_is_apparent = true:suggestion
+csharp_style_prefer_readonly_struct = true:suggestion
+csharp_style_prefer_readonly_struct_member = true:suggestion
 
 dotnet_sort_system_directives_first = true
 dotnet_style_qualification_for_field = false:warning
@@ -35,6 +46,7 @@ dotnet_style_predefined_type_for_locals_parameters_members = true:warning
 dotnet_style_predefined_type_for_member_access = true:warning
 dotnet_style_require_accessibility_modifiers = for_non_interface_members:warning
 dotnet_style_readonly_field_modifier = true:suggestion
+dotnet_style_prefer_collection_expression = when_types_loosely_match:suggestion
 
 [*.md]
 trim_trailing_whitespace = false

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,7 +6,11 @@
 
 - 
 
-## Test Plan
+## Checklist
 
-- [ ] Tests pass locally
+- [ ] Tests pass locally (`./build.sh Test`)
+- [ ] Code formatting verified (`./build.sh Format`)
 - [ ] No new warnings introduced
+- [ ] Public API changes reflected in `PublicAPI.Unshipped.txt` (if applicable)
+- [ ] `packages.lock.json` updated if dependencies changed (`dotnet restore`)
+- [ ] Version bump in `Directory.Build.props` (if this is a release PR)

CONTRIBUTING.md

@@ -0,0 +1,55 @@
+# Contributing
+
+Thank you for your interest in contributing to this project!
+
+## Prerequisites
+
+- [.NET SDK](https://dotnet.microsoft.com/download) (version specified in `global.json`)
+- [Node.js 22+](https://nodejs.org/) (only for documentation)
+
+## Getting Started
+
+```bash
+git clone <repo-url>
+cd dotnet.CI.template
+dotnet restore
+dotnet build
+dotnet test
+```
+
+## Build System
+
+This project uses [NUKE](https://nuke.build/) for build automation. All build logic lives in `build/BuildTask.*.cs`.
+
+Common targets:
+
+| Target | Description |
+|--------|-------------|
+| `./build.sh Build` | Compile all projects |
+| `./build.sh Test` | Run tests |
+| `./build.sh CoverageReport` | Generate coverage report with threshold enforcement |
+| `./build.sh Format` | Verify code formatting |
+| `./build.sh Pack` | Create NuGet packages |
+| `./build.sh ShowVersion` | Display current version |
+
+## Development Workflow
+
+1. Create a feature branch from `main`.
+2. Make your changes, ensuring tests pass and code formatting is correct.
+3. Submit a pull request targeting `main`.
+
+## Code Style
+
+- Code style is enforced by `.editorconfig` and `dotnet format`.
+- Run `./build.sh Format` locally before committing.
+- All warnings are treated as errors (`TreatWarningsAsErrors`).
+
+## Versioning
+
+- Version is managed via `VersionPrefix` in `Directory.Build.props`.
+- A release is triggered automatically when a new version is pushed to `main` and the corresponding git tag does not yet exist.
+
+## Public API Changes
+
+- Packable projects track their public API surface via `PublicAPI.Shipped.txt` and `PublicAPI.Unshipped.txt`.
+- When adding or modifying public APIs, update `PublicAPI.Unshipped.txt` accordingly.