Documents solution folder structure for AI agents

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

AGENTS.md

@@ -0,0 +1,13 @@
+# SlpModularCms — Solution Structure
+
+This is the canonical description of the solution layout. It is mirrored into `CLAUDE.md`, `.junie/guidelines.md`, and `.github/copilot-instructions.md` so every AI coding agent (Claude, Junie, GitHub Copilot, etc.) picks it up. If you update this file, update those too.
+
+This solution is organized into three top-level Solution Folders. Any agent working in this repo should respect this layout when adding, moving, or referencing projects.
+
+- **Application** — Development versions of the applications: the Master and Slave APIs, plus a nested **Modules** solution folder containing the individual CMS modules (e.g. `SlpModularCms.Modules.Master`, `SlpModularCms.Modules.Identity`, `SlpModularCms.Modules.Availability`).
+- **Tests** — Test projects mirroring the structure of **Application**, with the same nested **Modules** grouping — except the Slave API, which has no test project.
+- **Clients** — The actual buildable/shippable API projects (e.g. `SlpModularCms.Api`).
+
+When adding a new project:
+- A new module goes under `Application/Modules`, with its test project under `Tests/Modules`.
+- A new deployable API/client goes under `Clients`.