Commit yarn 4 in the tree and install with --immutable (closes #149)

Follows sneak's ruling at
#149 (comment). This
repository now carries yarn 4.18.1 under .yarn/releases, and whichever
yarn the distribution packages hands the work to it. yarn.lock is
converted once, and script/bootstrap installs with --immutable instead
of stopping on yarn's major version. Node on Alpine is nodejs-current.
Every Dockerfile stage that needs yarn also copies .yarnrc.yml and
.yarn/releases/. The policies and both checklists say so, with the
steps for converting and for a new repository.

Model: opus-5-5

Dockerfile

@@ -6,17 +6,20 @@
 # system packages. script/ passes a new value on every build. That value
 # reruns every RUN step below the ARG line, and the install step keeps
 # its layer. node and yarn come from the distribution, which is trusted,
-# so they need no version and no hash.
+# so they need no version and no hash. That yarn hands the work to the
+# release file in .yarn/releases/, so every stage copies that file and
+# .yarnrc.yml before it runs script/bootstrap.
 #
 # alpine 3.24.3, 2026-11-04
 FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a AS lint
-RUN apk add --no-cache git make nodejs yarn
+RUN apk add --no-cache git make nodejs-current yarn
 ARG CACHEBUST
 
 WORKDIR /app
 
 COPY script/ script/
-COPY package.json yarn.lock ./
+COPY package.json yarn.lock .yarnrc.yml ./
+COPY .yarn/releases/ .yarn/releases/
 RUN script/bootstrap
 
 COPY . .
@@ -27,13 +30,14 @@ RUN yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 #
 # alpine 3.24.3, 2026-11-04
 FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a AS test
-RUN apk add --no-cache git make nodejs yarn
+RUN apk add --no-cache git make nodejs-current yarn
 ARG CACHEBUST
 
 WORKDIR /app
 
 COPY script/ script/
-COPY package.json yarn.lock ./
+COPY package.json yarn.lock .yarnrc.yml ./
+COPY .yarn/releases/ .yarn/releases/
 RUN script/bootstrap
 
 COPY . .
@@ -48,7 +52,7 @@ RUN echo "No tests defined."
 #
 # alpine 3.24.3, 2026-11-04
 FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a
-RUN apk add --no-cache git make nodejs yarn
+RUN apk add --no-cache git make nodejs-current yarn
 ARG CACHEBUST
 
 WORKDIR /app
@@ -59,7 +63,8 @@ COPY --from=test /app/package.json /dev/null
 # script/bootstrap finds the system packages above and installs the node
 # packages.
 COPY script/ script/
-COPY package.json yarn.lock ./
+COPY package.json yarn.lock .yarnrc.yml ./
+COPY .yarn/releases/ .yarn/releases/
 RUN script/bootstrap
 
 COPY . .