add AGENTS.md with repository-specific guidance for coding agents

[aero-oli]

Summary

Add a root AGENTS.md with repository-specific guidance for coding agents and AI-assisted contributors working in the Mission Planner repo.

Why

Mission Planner has a few repo-specific constraints that are easy for agents to get wrong: it is primarily a Windows/Visual Studio codebase, it uses submodules, the recommended build target for normal work is the MissionPlanner project, and there are two different plugin models (Plugins/ projects vs lowercase plugins/ runtime .cs scripts). A focused AGENTS.md should help agent-authored PRs stay smaller, safer, and easier to review.

What this adds

• setup/build guidance aligned with the existing README/dev docs
• a concise map of the main repo areas
• explicit guidance to prefer small, surgical diffs
• clearer expectations around validation and PR summaries
• repo-specific cautions for:
  • WinForms/UI changes
  • MAVLink / CurrentState / units / fallback logic
  • shared code in ExtLibs/
  • the distinction between compiled plugins and runtime plugin scripts

Impact

Documentation only.

This does not change runtime behavior, build outputs, packaging, dependencies, or release artifacts.

Validation

Drafted from a review of:

• the current repo README/build instructions
• the Mission Planner developer build page
• recent merged/open PR title/body patterns
• public forum guidance around Mission Planner plugins

The file is intended to improve the quality and mergeability of future agent-authored changes, not to replace maintainer judgement.