Make a missing CHECK_EPOCH fail the build (closes #91)

PR #89 stopped script/cibuild replaying cached check layers, but left a
gap: a bare `docker build .` with no --build-arg still faked. An unset
ARG is an empty string, an empty string is a stable cache key, and the
check layers replay from it. That gap mattered because REPO_POLICIES.md
names `docker build .` verbatim as a command that must be green, so the
documented command was the one that lied.

Both check stages now carry `RUN [ -n "$CHECK_EPOCH" ] || exit 1`
immediately under their own ARG. Failed steps are never cached, so this
fails on every invocation rather than once - a bare build now stops with
a named error instead of reporting a green it did not earn. Each stage
needs its own guard because ARG scope is per-stage; a gate-carrying stage
without one is a silent hole if ordering ever changes.

The check RUNs now reference the value (`echo "check epoch: ${CHECK_EPOCH}"
&& make <target>`), so the cache miss is contractual rather than resting
on BuildKit's current treatment of unreferenced ARGs, and the epoch is
visible in the build log.

The epoch becomes "$(date +%s%N)$$" so concurrent invocations in the same
second cannot collide. busybox silently drops %N and exits 0, so $$ is
what makes it correct there. The bare-assignment form is retained
deliberately: inlining the substitution into --build-arg would, under
set -eu, yield an empty and therefore constant epoch without aborting.

script/docker gets the same treatment - it is not the gate, but two
entrypoints disagreeing about whether the tree is green is its own
hazard, and local builds are almost always warm.

Verified by negative control rather than inspection: a bare build fails
twice consecutively here and succeeds twice on the parent commit, so the
change is demonstrably not a no-op. The builder-stage guard was fired
directly with a targeted probe build, since the lint stage otherwise
fails first and would leave it unexercised.

script/docker

@@ -9,7 +9,16 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build -t "$("$SCRIPT_DIR/projectname")" .
+    # Same CHECK_EPOCH contract as script/cibuild, for the same reason
+    # and with the same bare-assignment and `$$` requirements -- see the
+    # comments there. This script is not the CI gate, but a local build
+    # is almost always warm, so without this it would report a green the
+    # tree had not earned and the two entrypoints would disagree about
+    # whether the tree is clean. The Dockerfile now refuses to build
+    # without a non-empty value, so this is required, not optional.
+    epoch="$(date +%s%N)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"