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

script/lint-fix

@@ -5,27 +5,28 @@
 #
 # THIS IS A DEVELOPER CONVENIENCE AND NEVER A GATE. Nothing in
 # script/check, script/precommit or script/cibuild calls it, and no gate
-# reads its exit status. The gate is script/lint, which builds
-# Dockerfile.lint; run that afterwards to find out whether the tree is
-# actually clean.
+# reads its exit status. The gate is script/lint, which builds the lint
+# phase of the Dockerfile; run that afterwards to find out whether the
+# tree is actually clean.
 #
 # Unlike script/lint this cannot be a build step: a build step writes
 # into an image, and fixes have to land in the worktree. So it runs the
 # same pinned image as a container with the tree bind-mounted, which
 # means it needs a LOCAL docker daemon -- a remote daemon has no access
 # to these files, and this script will appear to do nothing there. The
-# image reference is parsed out of Dockerfile.lint's FROM line, so the
+# image reference is parsed out of the lint phase's FROM line, so the
 # autofixer is always the same version as the linter that gates; fixes
 # written by a different version are not necessarily fixes for the
 # version that decides.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
-DOCKERFILE="$ROOT/Dockerfile.lint"
+DOCKERFILE="$ROOT/Dockerfile"
 
-# The image reference from Dockerfile.lint, tag and digest included.
+# The image reference from the FROM line ending `AS lint`, tag and
+# digest included.
 lint_image() {
-    awk '$1 == "FROM" { print $2; exit }' "$DOCKERFILE"
+    awk '$1 == "FROM" && $NF == "lint" { print $2; exit }' "$DOCKERFILE"
 }
 
 main() {
@@ -33,7 +34,7 @@ main() {
 
     image="$(lint_image)"
     if [ -z "$image" ]; then
-        echo "lint-fix: no FROM line found in $DOCKERFILE" >&2
+        echo "lint-fix: no FROM ... AS lint line found in $DOCKERFILE" >&2
         exit 1
     fi