Add repository guidance for agents

[adamdriscoll]

This adds a repository-level guide so future agents can quickly understand how PSCommander is structured, built, and safely validated. The project has several Windows integration points, so capturing the architecture and runtime cautions should reduce accidental changes to user state during future work.

Summary

• Adds AGENTS.md with project overview, key files, build commands, and validation guidance.
• Documents the PowerShell module plus WPF service flow for adding new configuration features.
• Calls out Windows-only behavior and cautions around registry, file association, hot key, named pipe, and desktop integration testing.

Testing

• Not run; documentation-only change.