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. A
host without Go gets the go.mod version from script/install-go in
.tool/go, which bootstrap, the Makefile, fmt, fmt-check, precommit and
release add to PATH; fmt-check skips .tool. The image takes its version
from the VERSION build arg or git describe, dev without .git. This
repo's own entries follow the canonical content in .gitignore and
.editorconfig. The golangci-lint v2.14.0 findings are fixed. The rules
in CLAUDE.md move into AGENTS.md. IsDevVersion counts "unknown".

Model: opus-5-5

cmd/vaultik/makefile_test.go

@@ -1,6 +1,8 @@
 package main_test
 
 import (
+	"os"
+	"path/filepath"
 	"regexp"
 	"slices"
 	"strings"
@@ -84,14 +86,48 @@ func TestBuildTargetBuildsTheBinary(t *testing.T) {
 		"`make build` must depend on the rule that builds the binary")
 }
 
-// readMakefile returns the contents of the repository's Makefile. The
-// root is located by the shared walk in lintdocker_test.go.
+// readMakefile returns the contents of the repository's Makefile.
 func readMakefile(t *testing.T) string {
 	t.Helper()
 
 	return readRepoFile(t, "Makefile")
 }
 
+// readRepoFile reads a file by its path relative to the repository
+// root.
+func readRepoFile(t *testing.T, name string) string {
+	t.Helper()
+
+	//nolint:gosec // G304: the path is a constant relative to this repo
+	contents, err := os.ReadFile(filepath.Join(repoRoot(t), name))
+	require.NoError(t, err)
+
+	return string(contents)
+}
+
+// repoRoot returns the repository root. The test binary runs with its
+// package directory as the working directory, so the root is found by
+// walking up until the module file appears.
+func repoRoot(t *testing.T) string {
+	t.Helper()
+
+	dir, err := os.Getwd()
+	require.NoError(t, err)
+
+	for {
+		_, err = os.Stat(filepath.Join(dir, "go.mod"))
+		if err == nil {
+			return dir
+		}
+
+		parent := filepath.Dir(dir)
+		require.NotEqual(t, dir, parent,
+			"walked to the filesystem root without finding a go.mod")
+
+		dir = parent
+	}
+}
+
 // phonyTargets returns every name declared phony, across all .PHONY
 // lines.
 func phonyTargets(makefile string) []string {