Re-vendor the canonical files from sneak/prompts at e3f79c4 (closes #95)

Every vendored file, REPO_POLICIES.md and every model script is the
copy at sneak/prompts e3f79c4, with this repository's own entries kept
after the canonical content. Lint and test are phases of the Dockerfile
that write no image. Every build in script/ passes a new CACHEBUST
value, which every stage declares first, so every RUN step runs again;
the steps that compile keep Go's build cache, and both go test runs
pass -count=1. make test runs the suite under the race detector as
nobody. Dockerfile.lint, script/verify-lint-image-pin and
make test-race are gone. Prettier runs on the host.
.claude/settings.json is deleted.

Deviation: the set comes from e3f79c4 on next rather than dd4027b.
Judgement call: the test phase mounts the cache at nobody's
/tmp/.cache/go-build, owned by nobody.

Model: opus-5-5

script/cibuild

@@ -1,19 +1,32 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Gitea workflow runs this on
-# push. The Dockerfile runs every gate make check runs, as build steps,
-# so a successful build means the repo is green.
-#
-# Without a fresh CHECK_EPOCH, a rebuild of an unchanged checkout serves
-# the gate layers from cache and passes having run none of them. The
-# process id goes in with the epoch so two runs started in the same
-# second still differ.
+# script/cibuild: run the CI build. It bootstraps first: a CI runner
+# checks out and runs this and nothing else, and script/fmt-check runs
+# the formatter on the host, which a pristine checkout cannot do.
+# A new CACHEBUST value reruns every RUN step, since a cached gate phase
+# is a check that did not run; unlike --no-cache, it keeps Go's build cache.
+# This relies on every stage of the Dockerfile declaring ARG CACHEBUST
+# first; a stage without that line is served from cache on an unchanged
+# tree, with no warning.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build --build-arg CHECK_EPOCH="$(date +%s)-$$" .
+    "$SCRIPT_DIR/bootstrap"
+    "$SCRIPT_DIR/check"
+    # The version and the tag each get their own line: a failing
+    # command substitution inside an argument does not trip `set -e`,
+    # so the inline form degrades silently to an empty constant. The
+    # VERSION build argument takes precedence over the version a build
+    # stage derives from the .git in the context.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    tag="$("$SCRIPT_DIR/projectname")"
+    docker build --build-arg CACHEBUST="$(date +%s)-$$" \
+        --build-arg VERSION="$version" \
+        -t "$tag" .
 }
 
 main "$@"