Re-vendor the canonical files from sneak/prompts at dd4027b (closes #213)

Linting and testing become the lint and test phases of the Dockerfile,
and the build stage depends on both. Dockerfile.lint, CHECK_EPOCH and
the tests that checked them are removed. Every docker build in script/
passes --no-cache, and script/cibuild runs script/bootstrap first. The
image takes its version from the VERSION build arg or git describe, and
still stamps the commit and its date from .git. The golangci-lint
v2.14.0 findings are fixed in the code. The rules in CLAUDE.md move into
AGENTS.md. IsDevVersion now counts "unknown", the version script/docker
stamps outside a git checkout.

Model: opus-5-5

Makefile

@@ -40,9 +40,10 @@ setup:
 check:
 	@script/check
 
-# Run tests only. This runs the ENTIRE suite -- there is no separate
-# integration target and no build-tagged subset held back. In
-# particular internal/vaultik/integration_test.go, which does full
+# Run tests only, by building the test phase of the Dockerfile. This
+# runs the ENTIRE suite -- there is no separate integration target and
+# no build-tagged subset held back. In particular
+# internal/vaultik/integration_test.go, which does full
 # chunk -> pack -> encrypt -> upload -> restore round-trips, runs here.
 # A `test-integration` target used to exist and was removed: no file in
 # the repo carried a build tag, so `-tags=integration` selected nothing
@@ -87,17 +88,17 @@ clean:
 	go clean
 
 # Install dependencies. The linter is deliberately not installed here:
-# script/lint lints by building Dockerfile.lint, whose FROM line is the
-# single source of truth for the linter version. A second, separately
-# pinned copy on PATH could drift from it and make a local `make lint`
-# disagree with CI.
+# script/lint lints by building the lint phase of the Dockerfile, whose
+# FROM line is the single source of truth for the linter version. A
+# second, separately pinned copy on PATH could drift from it and make a
+# local `make lint` disagree with CI.
 deps:
 	go mod download
 
-# Run tests with coverage. -count=1 for the same reason script/test
-# uses it: without it an unchanged package is served from Go's test
-# result cache, and a coverage profile assembled from cached results
-# describes a run that did not happen.
+# Run tests with coverage, on the host. -count=1 because without it an
+# unchanged package is served from Go's test result cache, and a
+# coverage profile assembled from cached results describes a run that
+# did not happen.
 test-coverage:
 	go test -v -count=1 -coverprofile=coverage.out ./...
 	go tool cover -html=coverage.out -o coverage.html