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/bootstrap

@@ -48,12 +48,12 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
-# Docker is a hard requirement, not a nice-to-have: script/lint lints by
-# building Dockerfile.lint, whose digest-pinned golangci-lint image is
-# the only place the linter runs, and script/check and script/precommit
-# both run script/lint. A bootstrap that prints "bootstrap complete" on a
-# machine where `make check` cannot run is a false success, so this fails
-# instead.
+# Docker is a hard requirement, not a nice-to-have: script/lint and
+# script/test build the lint and test phases of the Dockerfile, the only
+# place the linter and the tests run, and script/check and
+# script/precommit both run them. A bootstrap that prints "bootstrap
+# complete" on a machine where `make check` cannot run is a false
+# success, so this fails instead.
 #
 # Installing docker from here was considered and rejected: it needs root,
 # a running daemon, and on macOS a GUI cask, so an attempt would itself
@@ -80,15 +80,15 @@ bootstrap: FAILED - $reason.
 
 Docker is required to develop this repo. Without it these do not work:
 
-  script/lint      builds Dockerfile.lint, which runs the linter as a
-                   build step in a digest-pinned golangci-lint image.
-                   That FROM line is the single source of truth for the
-                   linter version
-  script/check     runs script/lint
+  script/lint      builds the lint phase of the Dockerfile, which runs
+                   the linter as a build step in a digest-pinned
+                   golangci-lint image
+  script/test      builds the test phase of the Dockerfile
+  script/check     runs script/test and script/lint
   script/precommit runs script/check, so commits are blocked by the
                    pre-commit hook installed by script/setup
-  script/cibuild   builds Dockerfile.lint and Dockerfile, which is what
-                   CI runs
+  script/cibuild   runs script/check and builds the image, which is
+                   what CI runs
 
 Install docker (and start the daemon, checking DOCKER_HOST and your
 group membership), then re-run script/bootstrap. golangci-lint on PATH
@@ -108,11 +108,12 @@ main() {
     if missing go; then pkg_install go golang go go; fi
 
     # golangci-lint is deliberately NOT installed: script/lint lints by
-    # building Dockerfile.lint, whose digest-pinned image is the only
-    # place the linter runs, so whatever a package manager happens to
-    # ship would only be a shadow of the pinned version that could drift
-    # from CI. Nothing on the host is ever used as a linter, at any
-    # version, so installing one here would buy nothing.
+    # building the lint phase of the Dockerfile, whose digest-pinned
+    # image is the only place the linter runs, so whatever a package
+    # manager happens to ship would only be a shadow of the pinned
+    # version that could drift from CI. Nothing on the host is ever used
+    # as a linter, at any version, so installing one here would buy
+    # nothing.
 
     # goreleaser, at the version pinned by script/install-goreleaser and
     # verified against a hardcoded sha256. Package managers are not used