Add CLAUDE.md: local checks in pinned Docker images

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Local builds and checks
+
+Everything runs in Docker. Nothing is installed on the host: no compiler, mbedTLS or
+shellcheck. shellcheck is pinned to a release and the build image to a Debian release
+(bookworm, mbedTLS 2.28). The `docker run` commands pass `--pull never`, so a check never
+pulls a different version than the last run. On a fresh machine, pull the images once:
+
+```sh
+docker pull koalaman/shellcheck:v0.10.0
+docker pull debian:bookworm-slim
+```
+
+The CI `lint` job checks:
+
+```sh
+# shell scripts
+docker run --rm --pull never -v "$PWD:/mnt" -w /mnt koalaman/shellcheck:v0.10.0 \
+  -S warning install.sh uninstall.sh ci/build.sh ci/assemble.sh tests/run.sh tests/suite.sh
+
+# native build (mbedTLS 2.28 from Debian bookworm), then lint the init script the binary writes
+docker run --rm --pull never -v "$PWD:/src:ro" debian:bookworm-slim sh -c \
+  'apt-get update -qq && apt-get install -y -qq --no-install-recommends gcc make libc6-dev libmbedtls-dev >/dev/null &&
+   cp -r /src/src /tmp/s && make -s -C /tmp/s EXTRA_CFLAGS=-Werror >&2 && /tmp/s/cerberus init-script -' > /tmp/cerberus.init
+docker run --rm -i --pull never koalaman/shellcheck:v0.10.0 -S warning -e SC2155 -s sh - < /tmp/cerberus.init
+```
+
+Use `koalaman/shellcheck:v0.10.0`, not `:stable`: the `stable` tag moves between releases.
+`make -s` sends its size line to stderr (`>&2`); without that it lands in the script and the
+lint fails with SC2275.
+
+A pull request that changes `src/`, the installers, the `Makefile` or `ci/` also needs a new
+`CERBERUS_VERSION` in `src/cerberus.c` and its section in `CHANGELOG.md`; CI checks that with
+`.github/check-version.sh`, which runs the same locally against `origin/master`.
+
+Tests and release binaries already run in containers through the repository's own scripts:
+
+```sh
+tests/run.sh                                            # Alpine (default image in tests/run.sh), musl + mbedTLS 3.x
+CERBERUS_TEST_IMAGE=debian:bookworm-slim tests/run.sh   # glibc + mbedTLS 2.28
+ci/build.sh arm                                         # static musl binary; all four without arguments
+```
+
+## Rules
+
+- Never build on the host. `make -C src` outside a container leaves a host-built `src/cerberus`
+  behind.
+- Local builds are for development only. Release binaries come from CI, never from a local build.