feat(loader): add cameleer-runtime-loader image (busybox + entrypoint)

Init container that fetches the deployable JAR from a signed URL into the
shared /app/jars/ volume before the main runtime container starts. Pairs
with the controller (Task 7) and DockerRuntimeOrchestrator (Task 10).

- Dockerfile: busybox:1.37-musl, non-root USER (UID 1000)
- entrypoint.sh: POSIX sh, set -eu, required env vars (ARTIFACT_URL,
  ARTIFACT_EXPECTED_SIZE), wget with retries/timeout, size verification
- README: build instructions and runtime contract

Smoke-tested locally (docker build + happy-path fetch + size-mismatch).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cameleer-runtime-loader/Dockerfile

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+# Tiny init-container image. No app code, no shell-injection surface — script
+# only sees env vars set by the orchestrator.
+FROM busybox:1.37-musl
+
+# Run as non-root (UID 1000 inside the container; with userns_mode this is
+# remapped to host UID ~101000 — fully unprivileged on the host).
+RUN adduser -D -u 1000 loader
+
+COPY entrypoint.sh /usr/local/bin/loader
+RUN chmod +x /usr/local/bin/loader
+
+USER loader
+WORKDIR /app
+ENTRYPOINT ["/usr/local/bin/loader"]

cameleer-runtime-loader/README.md

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+# cameleer-runtime-loader
+
+Init container that fetches the deployable JAR into a shared volume before the
+main runtime container starts. Pairs with `DockerRuntimeOrchestrator` /
+(future) K8s init-container deploys.
+
+## Build
+
+    docker build -t gitea.siegeln.net/cameleer/cameleer-runtime-loader:<tag> .
+    docker push gitea.siegeln.net/cameleer/cameleer-runtime-loader:<tag>
+
+## Contract
+
+- Env: `ARTIFACT_URL` (signed download URL), `ARTIFACT_EXPECTED_SIZE` (bytes).
+- Volume: writes `/app/jars/app.jar`.
+- Exit 0 on success; non-zero on fetch/size failure.
+- Runs as UID 1000 (loader user), drops all caps, read-only rootfs except `/app/jars`.
+
+See `docs/superpowers/plans/2026-04-27-init-container-jar-fetch.md`.

cameleer-runtime-loader/entrypoint.sh

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+#!/bin/sh
+# cameleer-runtime-loader: fetches one JAR from a signed URL into the shared
+# /app/jars/ volume, verifies size, exits. Runs in the same hardened sandbox as
+# the main container (cap_drop ALL, read-only rootfs, etc.) — only /app/jars/
+# is writeable.
+set -eu
+
+: "${ARTIFACT_URL:?ARTIFACT_URL is required}"
+: "${ARTIFACT_EXPECTED_SIZE:?ARTIFACT_EXPECTED_SIZE is required}"
+
+OUT=/app/jars/app.jar
+mkdir -p /app/jars
+
+echo "loader: fetching artifact (expected $ARTIFACT_EXPECTED_SIZE bytes)"
+# -q quiet, -O output, --tries=3 retry transient network blips,
+# --timeout=30 cap stalls. wget exits non-zero on HTTP >=400.
+wget -q --tries=3 --timeout=30 -O "$OUT" "$ARTIFACT_URL"
+
+actual=$(wc -c < "$OUT")
+if [ "$actual" -ne "$ARTIFACT_EXPECTED_SIZE" ]; then
+    echo "loader: size mismatch — expected $ARTIFACT_EXPECTED_SIZE, got $actual" >&2
+    exit 2
+fi
+
+echo "loader: artifact written to $OUT ($actual bytes)"