Add AGENTS.md instructions file to improve AI assistance (#1486)

* Make agent assisted commits add a changelog entry
* Enable developers to ask copilot to update the changelog
  for them

Author: jasonleenaylor

.vscode/settings.json

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+"chat.useAgentsMdFile": true

AGENTS.md

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+# LibPalaso Copilot Instructions
+
+You are an expert C# developer assisting with the `sillsdev/libpalaso` repository. Follow these critical guidelines for all code generation.
+
+## 1. Code Standards & Quality
+- **Modern C#:** Prefer modern C# syntax (e.g., pattern matching, switch expressions, file-scoped namespaces) unless maintaining legacy consistency.
+- **Null Safety:** Strictly adhere to Nullable Reference Types. Explicitly handle potential nulls; do not suppress warnings with `!` unless absolutely necessary.
+- **Cross-Platform:** Remember that LibPalaso runs on Windows and Linux (Mono/.NET). Avoid Windows-specific APIs (like `Registry` or hardcoded `\` paths) unless wrapped in OS checks.
+
+## 2. Testing
+- **Framework:** Use **NUnit** for all unit tests.
+- **Mocking:** Use **Moq** for mocking interfaces when necessary.
+- **Structure:** When writing tests, follow the `Given_When_Then` or `State_Action_Result` naming convention for clarity.
+
+## 3. Documentation & Process (Critical)
+- **Update the Changelog:** If the suggested code changes functionality, fixes a bug, or adds a feature, you **must** generate an update for `CHANGELOG.md`.
+    - Look for the `## [Unreleased]` section at the top of the changelog.
+    - Insert a bullet point under the appropriate subsection based on the type of change:
+        - `### Added` - for new features, methods, classes, or capabilities
+        - `### Fixed` - for bug fixes and corrections
+        - `### Changed` - for changes in existing functionality, including BREAKING CHANGES
+        - `### Deprecated` - for soon-to-be removed features (mark with `[Obsolete]` attribute)
+        - `### Removed` - for features that have been completely removed
+        - `### Security` - for security-related fixes or improvements
+    - If the subsections do not exist under `[Unreleased]`, create them as needed.
+    - Format: `- **[Scope]** Description of the change.` (where Scope is the affected library/namespace)
+    - For breaking changes, prefix with `BREAKING CHANGE:` in the description.