Adds AGENTS.md as the canonical description of the Application/Tests/Clients
solution layout, mirrored into CLAUDE.md, .junie/guidelines.md, and
.github/copilot-instructions.md so Claude, Junie, and Copilot all pick it up.
Also un-ignores .junie/ so guidelines.md is tracked.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wi5qHAuq8UbzN4NLUFeKkJ
