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/keios-port-themes-payments.md

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+---
+title: keios.eu port — server-rendered themes and payments
+trigger_condition: Płytarium v1 is running in production on SummerCMS Go (vue-fonoteka-app unchanged).
+planted_date: 2026-09-16
+---
+
+# keios.eu port: themes and payments
+
+Second port target after Płytarium. keios.eu is the smallest Golem15 project that has both a Twig theme and a payment flow.
+
+| Aspect | keios.eu |
+|---|---|
+| Golem15 plugins | apparatus, dualformfield, faq, golem, journal, knobwidget, paymentgateway, pgstripe, quote, sitemap, translate, user, websockets |
+| Theme | `wn-keioseu-theme`, 14 template files |
+| Plugin PHP lines | ~74k |
+
+## What it adds to SummerCMS
+
+- Theme engine with Twig-like syntax (`pongo2` per research, used only for themes) and components dropped into templates.
+- `paymentgateway` and `pgstripe` port. Stripe Go SDK is official.
+- `journal`, `faq`, `quote` as classic content plugins with admin forms.
+
+## Related
+
+- `../notes/v1-target-plytarium.md`

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

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+---
+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)