Add repository guidance for coding agents

[schaubh]

Summary

• Add a root-level AGENTS.md describing the BSK-SDK architecture and public plugin contract.
• Document branch and PR expectations, generated/vendored boundaries, and supported build and test workflows.
• Capture lessons from previous changes around SWIG runtime compatibility, CMake scope, generated message bindings, version checks, and downstream plugin testing.
• Add a focused checklist for future branch and PR reviews.

Motivation

BSK-SDK has several integration points where a locally successful change can
still break installed wheels or downstream plugins. This guidance consolidates
the repository-specific knowledge needed to make and review changes safely,
especially around Basilisk synchronization, CMake/SWIG behavior, and the
custom-atm-plugin compatibility example.

Impact

Documentation and contributor guidance only. There are no runtime, packaging,
CMake, dependency, or downstream API changes.

The Basilisk submodule remains unchanged at v2.11.0.

Verification

Not run because this is a documentation-only change. The branch contains only
the new AGENTS.md file.

[chatgpt-codex-connector]

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