✨ Adds project guidelines and conventions for Github Copilot

Introduces detailed instructions for ASP.NET API, C#, Docker, MSBuild,
testing, and library creation. These guidelines aim to standardize
development practices and improve code quality across the project.

.github/copilot-instructions.md

@@ -0,0 +1,12 @@
+# Project Guidelines
+
+## Behaviour
+
+- **Tone**: direct and concise — minimise prose, maximise actionable output.
+- **Research first, act second** — always consult official documentation, codebase context, and project conventions **before** proposing or implementing changes. Never guess, assume API shapes, or rely on trial-and-error. If documentation is unclear or unavailable, say so and ask for guidance.
+- On ambiguous requests, **ask for clarification** — prefer offering interactive options over waiting for a new prompt.
+- Briefly explain every suggestion: what it does, why, and how it fits the architecture.
+- Work in parallel whenever possible.
+- **Prefer subagents** for multi-step read-only operations (searches, file reads, doc fetches) to conserve main conversation context.
+- When goals conflict (e.g., performance vs readability), **favour performance** unless readability is significantly harmed.
+- **Persist learnings** — when you discover a pattern, resolve a non-obvious issue, or make a decision worth remembering, save it to memory. Use repo memory for project-specific facts and user memory for general preferences. Keep memory up to date when instructions or project conventions change.