Add Copilot agent instructions

.github/copilot-instructions.md

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+## ALICE data Quality Control framework and Modules
+
+This repository contains the ALICE O<sup>2</sup> data Quality Control (QC) framework and modules.
+It is divided into `Framework`, maintained by the framework developers and `Modules`, which are mostly developed by detector experts.
+
+### Copilot agent instructions
+
+- All the code should apply the [ALICE O2 Coding Guidelines](https://github.com/AliceO2Group/CodingGuidelines).
+See [Formatting tool](https://github.com/AliceO2Group/CodingGuidelines?tab=readme-ov-file#formatting-tool) for details how to set up code formatter.
+Do not attempt to install `clang` with `aliBuild`, use the one you have available on your system.
+- Unless you are running on a developer's machine and you were provided specific instructions, do not attempt to build the code or run tests. Quality Control uses a large number of external dependencies, which are not available in the default Copilot environment on GitHub.
+- When working on the C++ code-base, use the C++20 standard.
+- When modifying class definitions for which a ROOT dictionary is generated, remember to update their version in macros such as `ClassDefOverride` (e.g. `ClassDefOverride(MonitorObject, 15)` -> `ClassDefOverride(MonitorObject, 16)`).
+- Avoid changes which are not relevant to the current task.
+- When adding a new feature, write a unit test if feasible. Unit tests should use catch2.
+- When adding a new feature, extend the documentation accordingly and make sure that Tables of Contents are updated.
+- When providing a fix, explain what was causing the issue and how it was fixed.
+- When adding new code, make sure that necessary headers are included. Likewise, when removing code, remove the corresponding headers if they are not needed anymore.
+- When dealing with the code in `Modules`, prefer minimal changes which do not break the existing functionality.