Re-vendor the canonical files from sneak/prompts at dd4027b (closes #95)

The shared files are the copies at sneak/prompts commit dd4027b, with this
repository's own entries kept after them. golangci-lint is v2.14.0 and
raises no findings. Lint and test are phases of the Dockerfile, built by
script/lint and script/test; the tests run under the race detector as
nobody, so Dockerfile.lint, script/verify-lint-image-pin and make
test-race are gone. Every docker build in script/ passes --no-cache in
place of the old cache-busting build argument, and script/cibuild runs
script/bootstrap and script/check before the image build.
.claude/settings.json is deleted.

Deviation: the workflow keeps fetch-depth: 0 for the tag-derived version.
Deviation: .gitignore keeps the scan database patterns.
Deviation: prettier still runs in Docker, not on the host.
Over the cap: make test takes about 60 seconds on this host.

Model: opus-5-5

script/docker

@@ -1,9 +1,8 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
-# The tag comes from script/projectname. CHECK_EPOCH is passed for the
-# same reason script/cibuild passes it: without a fresh value an
-# unchanged tree is served from cache and this exits 0 having run no
-# gate.
+# Identical in all repos; the tag comes from script/projectname.
+# --no-cache because the gate phases the final stage depends on are RUN
+# steps, and a cached one is a check that did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -11,10 +10,15 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build \
-        --build-arg CHECK_EPOCH="$(date +%s)-$$" \
-        -t "$("$SCRIPT_DIR/projectname")" \
-        .
+    # 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"
+    docker build --no-cache \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"