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, which
now fetches apt package lists so a fresh CI runner can install Go. 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

internal/globals/globals.go

@@ -72,9 +72,12 @@ func New() (*Globals, error) {
 // safe reading of "we could not establish that this is a release" is
 // that it is not one. The Makefile refuses to build at all in that
 // case; this is the second line of defence, for a binary linked by
-// something other than the Makefile.
+// something other than the Makefile. "unknown" counts for the same
+// reason: script/docker and script/cibuild stamp it when the host has
+// no git checkout.
 func IsDevVersion(v string) bool {
-	if v == "" || v == DevVersion || strings.HasPrefix(v, DevVersion+"-") ||
+	if v == "" || v == "unknown" || v == DevVersion ||
+		strings.HasPrefix(v, DevVersion+"-") ||
 		strings.HasSuffix(v, "-dirty") {
 		return true
 	}

internal/globals/globals_test.go

@@ -74,6 +74,9 @@ func TestIsDevVersion(t *testing.T) {
 		// as one. The Makefile refuses to build when script/version
 		// yields nothing; this covers a binary linked some other way.
 		{"", true},
+		// What script/docker and script/cibuild stamp when the host
+		// has no git checkout.
+		{"unknown", true},
 	}
 
 	for _, tc := range cases {