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/docker

@@ -1,15 +1,8 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
-# The tag comes from script/projectname. Unlike the canonical copy in
-# sneak/prompts, it passes a fresh CHECK_EPOCH instead of --no-cache, and
-# COMMIT and COMMIT_DATE as well as VERSION.
-#
-# This builds the PRODUCT image only, and the product Dockerfile has no
-# lint stage: linting lives in Dockerfile.lint and is run by
-# script/lint. So a green here means `make fmt-check` and `make test`
-# passed and the image built -- it says nothing about lint. The gates
-# are script/check (which runs script/lint) and script/cibuild (which
-# builds both files).
+# Identical in all repos; the tag comes from script/projectname.
+# --no-cache because the gate phases the final stage depends on are RUN
+# steps, and a cached one is a check that did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -17,28 +10,14 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    # Same CHECK_EPOCH contract as script/cibuild, for the same reason
-    # and with the same bare-assignment and `$$` requirements -- see the
-    # comments there. This script is not the CI gate, but a local build
-    # is almost always warm, so without this it would report a green the
-    # tree had not earned and the two entrypoints would disagree about
-    # whether the tree is clean.
-    epoch="$(date +%s%N)$$"
-
-    # Version, commit and build date are computed here on the host and
-    # passed into the build, where they take precedence over what the
-    # build would derive from the .git in its context. VERSION comes
-    # from script/version, as in the Makefile, so the image reports the
-    # same string, -dirty included, that a local build of the same tree
-    # would.
-    version="$("$SCRIPT_DIR/version")"
-    commit="$(git rev-parse HEAD 2>/dev/null || echo unknown)"
-    commit_date="$(git show -s --format=%cs HEAD 2>/dev/null || echo unknown)"
-
-    docker build --build-arg CHECK_EPOCH="$epoch" \
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant. The VERSION build argument takes precedence over
+    # the version a build stage derives from the .git in the context.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    docker build --no-cache \
         --build-arg VERSION="$version" \
-        --build-arg COMMIT="$commit" \
-        --build-arg COMMIT_DATE="$commit_date" \
         -t "$("$SCRIPT_DIR/projectname")" .
 }