Create AGENTS.md with guidelines for developers

Added repository-specific instructions for agents working in ArduPilot/MissionPlanner, covering scope, environment, build validation, repository structure, plugin distinctions, and change discipline.

Author: aero-oli

AGENTS.MD

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+# AGENTS.md
+
+This file gives repository-specific instructions for agents working in `ArduPilot/MissionPlanner`.
+
+## Scope and priorities
+
+- Keep changes tightly scoped to the user request.
+- Prefer the smallest safe fix over broad refactors.
+- Do not “modernize” frameworks, package versions, project layout, or naming conventions unless the task is explicitly about that.
+- Avoid drive-by cleanup in this repo. It is large, Windows-first, and contains a mix of application code, libraries, plugins, generated/designer files, installers, scripts, drivers, and copied runtime assets.
+
+## Environment you should assume
+
+- Mission Planner is developed primarily on **Windows** and the README says **Visual Studio 2022** is the recommended build environment.
+- Use the repo-provided `vs2022.vsconfig` rather than guessing workloads/components.
+- Initialize submodules before building.
+- The repository contains an `ExtLibs/mono` submodule and the solution references projects under that tree.
+- VS Code may parse the codebase, but the README explicitly says it is **not sufficient for building** Mission Planner.
+
+## Build and validation
+
+Preferred validation path:
+
+1. Windows machine
+2. Visual Studio 2022
+3. `git submodule update --init`
+4. Open `MissionPlanner.sln`
+5. Build `MissionPlanner`
+
+If a properly provisioned Windows CLI environment is available, mirror CI behavior as closely as practical instead of inventing a new build flow.
+
+Useful CI facts:
+
+- CI builds `MissionPlanner.sln` in `Release | Any CPU` on Windows.
+- CI clears NuGet locals, initializes submodules, restores/builds with VSBuild, and archives output from `bin/Release/net461`.
+- Do not assume the app is a modern .NET 6 desktop app just because CI installs a .NET 6 SDK. The main app project targets .NET Framework and outputs to the legacy `net461` release folder.
+
+Minimum expectations before finishing a code change:
+
+- Build the solution, or at least the directly affected project(s), in the proper Windows/VS environment.
+- If your change touches existing tests or testable logic, run `MissionPlannerTests` as well.
+- If your change affects packaging, startup assets, plugin loading, drivers, or copied content, verify the expected files land in the output directory.
+
+## Repository map
+
+Treat the repo as a large Windows desktop solution with several layers:
+
+- `MissionPlanner.sln` — main solution.
+- `MissionPlanner.csproj` — primary WinForms application.
+- `MainV2.cs`, `GCSViews/`, `Controls/` — core UI/shell/view logic.
+- `ExtLibs/` — a large set of supporting libraries and shared components.
+- `MissionPlannerTests/` — test project included in the solution.
+- `Plugin/Plugin.cs` — base runtime plugin API.
+- `Plugins/...` and some plugin projects under `plugins/...` — compiled plugin projects included in the solution.
+- `plugins/example*.cs` and similar files — runtime plugin examples copied to output, not normal app compilation units.
+- `Updater/`, `wix/`, `Msi/`, `Drivers/` — installer/update/driver related areas.
+- `Scripts/`, `graphs/`, `NoFly/`, XML/JSON assets at repo root — runtime content used by the application.
+
+## Very important plugin distinction
+
+Do not treat every `plugins/*.cs` file as normal compiled application code.
+
+There are two different extension patterns in this repo:
+
+### 1) Compiled plugin projects
+
+The solution includes plugin projects such as:
+
+- `Plugins/Shortcuts/Shortcuts.csproj`
+- `Plugins/FaceMap/FaceMap.csproj`
+- `Plugins/OpenDroneID2/OpenDroneID_Plugin.csproj`
+- `Plugins/TerrainMakerPlugin/TerrainMakerPlugin.csproj`
+- `plugins/Dowding/Dowding.csproj`
+
+If you are changing one of these, treat it like normal C# project code and validate it accordingly.
+
+### 2) Runtime `.cs` plugin examples/scripts
+
+`MissionPlanner.csproj` explicitly copies many files like `plugins/example.cs`, `plugins/example2.cs`, etc. to the output as content, while excluding them from normal compilation.
+
+If you are changing one of these files:
+
+- Treat it as a runtime plugin/script artifact, not as a normal compile item.
+- Do not convert it into a normal app compile unit unless the task explicitly requires that architectural change.
+- Preserve the expected runtime loading model.
+
+## Plugin API expectations
+
+Runtime plugins derive from `MissionPlanner.Plugin.Plugin`.
+
+Important lifecycle/API facts from `Plugin/Plugin.cs`:
+
+- Required members: `Name`, `Version`, `Author`, `Init()`, `Loaded()`, `Exit()`.
+- Optional members include `SetupUI(...)`, `Loop()`, `NextRun`, and `loopratehz`.
+- `Loop()` runs on a **background thread shared with other plugins**.
+- `Exit()` is called on **plugin unload**, not on application exit.
+
+Implications for agents:
+
+- Be conservative with thread-affinity assumptions.
+- Do not touch WinForms controls from plugin background-loop code without proper marshaling.
+- Keep plugin load/unload behavior predictable.
+- Avoid adding blocking work in `Loop()`.
+
+## WinForms and designer files
+
+This is a WinForms-heavy repo.
+
+When editing UI code:
+
+- Prefer changing logic in the main `.cs` file unless the task actually requires designer changes.
+- Do not hand-edit `*.Designer.cs` or `.resx` casually.
+- If you must change designer-generated files, keep the related `.cs`, `.Designer.cs`, and `.resx` in sync.
+- Avoid accidental whitespace/reformat churn in designer files.
+
+## Project and asset handling rules
+
+The main project copies a large amount of runtime content to the output directory: drivers, XML/JSON, graphs, Python scripts, themes, KMZ files, plugin examples, and helper binaries.
+
+Because of that:
+
+- Do not rename, move, or delete runtime content files unless the task explicitly requires it.
+- Do not assume a file is unused just because it is not a normal C# compile item.
+- Be careful when touching `Drivers/`, `Scripts/`, `graphs/`, `NoFly/`, root XML/JSON, and copied helper executables.
+- Preserve existing copy-to-output behavior unless the task is specifically about packaging or deployment.
+
+## Legacy/compatibility posture
+
+This repo contains legacy .NET Framework, WinForms, installer, updater, plugin, and platform-compatibility concerns.
+
+Therefore:
+
+- Prefer compatibility-preserving changes.
+- Avoid introducing trendy abstractions or framework migrations as part of unrelated work.
+- Avoid broad namespace, file-layout, or solution-structure reorganizations.
+- Avoid replacing established project references/content behavior unless there is a clear repository-level reason.
+
+## Code style and change discipline
+
+- Follow the existing local style of the file you are editing.
+- Match naming and patterns already used nearby.
+- Keep diffs surgical.
+- Do not reformat large files just because you touched them.
+- Do not perform solution-wide analyzer or style cleanup as a side quest.
+
+## What to mention in your final change summary
+
+When you finish a change, include:
+
+- What you changed
+- Why those files were the right place to change
+- What you validated
+- Any remaining risk or area not validated in a real Windows/Visual Studio runtime environment
+
+## When to stop and reconsider
+
+Pause and reassess before making broad changes if the request would require any of the following:
+
+- Retargeting framework versions
+- Reworking build/packaging structure
+- Converting runtime plugin examples into compiled project code
+- Large-scale renames/moves of copied runtime assets
+- Cross-cutting WinForms designer regeneration across many screens
+
+In this repo, those are high-risk changes and should only happen when explicitly requested.