Add DESIGN.md and README overview

Capture the v1 architecture: domain model, geometry, SVG editor with
semantic zoom, REST API with version-guarded sync, service-layer seam
for future majordomo/executus agent tools, OIDC-first auth (Authentik),
and the phased roadmap.

Co-Authored-By: Claude Fable 5 <[email protected]>

README.md

@@ -1,2 +1,6 @@
 # pansy
 
+Self-hostable garden planner: drag beds, bags, and containers onto a real-scale field, click into them to place freeform plops of plants, and zoom out to see what's planted where. Go backend + React frontend, one static binary.
+
+See [DESIGN.md](DESIGN.md) for the architecture. Work is tracked in this repo's issues — start from the tracking epic.
+