Add .github/copilot-instructions.md for Copilot cloud agent onboarding

[Copilot]

Adds a copilot-instructions.md to give Copilot cloud agents enough context about this repo to work efficiently from first contact.

What's documented

• Project purpose & structure – annotated directory tree covering source, tests, analyzers, and CI
• Build/test commands – exact dotnet invocations, target frameworks, warnings-as-errors enforcement
• Core architecture – SlackWebApiClient facade, lazy-init API group pattern, IWebApiClient transport abstraction, Request/Response conventions
• Code conventions – naming rules, mandatory Newtonsoft.Json usage (no System.Text.Json), NullValueHandling.Ignore on optional props, one-class-per-file
• Extension patterns – step-by-step guides for adding API endpoints, API groups, event types, and message blocks, including interface wiring and test fixture requirements
• Test helpers – Utility assertion methods and fixture naming conventions (Web_, Events_, Blocks_, etc.)
• CI/CD – workflow triggers, deployment targets, commit-message skip flags
• Common pitfalls – polymorphic converter registration, internal-only API classes, Admin sub-group nesting