Initial commit: SummerCMS Go planning docs and research

Records the move from Scala to Go, the compiled-plugin decision, the
Płytarium port as the v1 target, and the Go ecosystem research that
backs the choice. No code yet.

.planning/seeds/wasm-extension-api.md

@@ -0,0 +1,23 @@
+---
+title: WASM extension API for untrusted third-party extensions
+trigger_condition: Core plugin API (models, form fields, components, events) has been stable for one full milestone and at least two compiled plugins extend each other through it.
+planted_date: 2026-09-16
+---
+
+# WASM extension API
+
+Decided during the opening exploration: primary plugins compile in (Caddy/xcaddy model), and a second, narrow extension surface runs sandboxed at runtime for third parties who cannot or should not rebuild the binary.
+
+## Shape
+
+- Runtime: `extism` on `wazero` (pure Go, no cgo).
+- Surface: deliberately narrow. Hooks on events, custom form field renderers, HTTP handlers under a namespaced prefix, read-only model queries through a host function. No direct DB access, no model extension.
+- Packaging: one `.wasm` per extension plus a manifest. Installable from the admin without rebuild.
+
+## Why not now
+
+Both RPC and WASM plugins fail the same way for a CMS core: the plugin surface (register models, extend other plugins' models, add form widgets, add components) is too wide to marshal across a boundary. Only once the compiled API settles do we know which narrow slice is worth exposing.
+
+## Related
+
+- `../notes/why-go-not-scala.md` (plugin decision)