Re-vendor the shared files from sneak/prompts at dd4027b (closes #504)

Fetches the shared files unchanged from sneak/prompts commit dd4027b and
adds .prettierignore; .dockerignore keeps this repository's anchored host
artifacts at its end.

Linting moves into the Dockerfile's lint phase on golangci-lint v2.14.0,
which also runs ESLint, and Dockerfile.lint is gone. Tests move into a test
phase on the Debian Go image. script/lint, test, docker and cibuild are the
model scripts, every docker build in script/ passes --no-cache, and the
.ci-fingerprint barrier is gone. The workflow no longer calls
script/ci-mark-superseded, so it and its tests are removed. make build
passes -trimpath and -s -w.

Model: opus-5-5

internal/middleware/loginguard_test.go

@@ -236,9 +236,9 @@ func TestLoginGuard_SemaphoreBoundsConcurrentVerifications(
 
 		// rendezvousDeadlock is the deadlock guard described below.
 		// It is orders of magnitude longer than any scheduling delay,
-		// so it never decides the result, and well inside script/test's
-		// 30s timeout, so a wedge fails on the assertion instead of
-		// blowing the package timeout.
+		// so it never decides the result, and well inside the 90s
+		// package timeout of the Dockerfile's test phase, so a wedge
+		// fails on the assertion instead of blowing that timeout.
 		rendezvousDeadlock = 5 * time.Second
 	)