Re-vendor the canonical files from sneak/prompts at dd4027b (closes #121)

The vendored files are copies from sneak/prompts dd4027b, with this
repository's own entries after the canonical content. golangci-lint is
v2.14.0. Lint and test are phases of the Dockerfile, which script/lint
and script/test build with --no-cache; Dockerfile.lint and
script/lint-darwin are gone, and the lint phase also checks the macOS
build. The tests run on the Debian Go image with cgo and the race
detector. script/cibuild bootstraps, runs script/check and builds the
image; CHECK_EPOCH and the memlock ulimit are gone. Go's build cache
stays in a cache mount, out of the test image's layer. Agent guidance
lives in AGENTS.md alone; the tool-specific file is gone.

Model: opus-5-5

script/test

@@ -1,22 +1,19 @@
 #!/bin/sh
-# script/test: run the test suite (vet first, then the tests with the race
-# detector; a verbose rerun on failure).
+# script/test: run the test suite. Testing is a phase of the Dockerfile
+# and this builds that phase alone, on the same terms as script/lint:
+# --target because a phase that is not the last stage is built only when
+# named, --no-cache because a cached test layer is a test that did not
+# run, and a tag so each build replaces the previous image.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    # CGO is required (Makefile exports this too)
-    export CGO_ENABLED=1
-    go vet ./...
-    # -count=1: run every test, never take a result from Go's test cache,
-    # which the Dockerfile keeps between builds. The rerun only prints
-    # details: `exit 1` keeps the script failing even if a flaky test
-    # passes on the second attempt.
-    go test -count=1 -timeout 30s -race -cover ./... || \
-        { echo "--- Rerunning with -v for details ---"; \
-          go test -count=1 -timeout 30s -race -v ./...; exit 1; }
+    docker build --no-cache \
+        --target test \
+        -t "$("$SCRIPT_DIR/projectname")-test" .
 }
 
 main "$@"