Build from local KamadoPool checkout via buildx build-context

Replaces the git-clone-at-SHA source stage with a 'FROM scratch
AS source' stage that COPYs from a named build context. The
Makefile passes '--build-context kamado=../KamadoPool' by
default; override with 'make KAMADO_SRC=/elsewhere'. No more
SHA placeholder, no GitHub dependency, no re-clone on every
build — and the image always reflects the working tree.

README.md

@@ -10,7 +10,11 @@ make
 
 This runs `deno` to bundle the embassy TypeScript procedures, builds a multi-arch OCI image via `docker buildx`, and packs everything into `kamado-pool.s9pk` using `start-sdk`.
 
-> **Note:** the Dockerfile clones the Kamado source from a pinned git SHA. Before the first build, set the `KAMADO_REPO` and `KAMADO_SHA` build args in [Dockerfile](Dockerfile) to a real pushed commit on the upstream repo.
+The build pulls KamadoPool source from a **local sibling checkout** (`../KamadoPool` by default) via a docker buildx named build context — no GitHub clone, no pinned SHA. If your checkout lives elsewhere, override it:
+
+```sh
+make KAMADO_SRC=/path/to/KamadoPool
+```
 
 ## Install