Make a missing CHECK_EPOCH fail the build instead of faking it (closes #91)

Adopt the four remaining upstream CHECK_EPOCH hardening items from
sneak/prompts #26, closing the gap #85 left open deliberately.

Fail closed on a missing value. Each check stage now asserts
`[ -n "$CHECK_EPOCH" ] || exit 1` before running anything. An unset ARG
is an empty string and an empty string is a stable cache key, so the
second and every later bare `docker build .` on an unchanged tree
replayed all three check layers, executed nothing, and still exited 0 --
and `docker build .` is the command REPO_POLICIES.md names verbatim as a
thing that must be green, so the documented command was precisely the
one that lied. Failed steps are never cached, which is what makes the
guard fire on every invocation rather than once.

Expand the epoch into each check command rather than leaving it a bare
declaration, so the cache miss does not depend on BuildKit's
unreferenced-ARG handling staying as it is, and so the value appears in
the build log where a reader can see the layer was keyed fresh.

Make the epoch unique per invocation rather than per second:
`epoch="$(date +%s%N)$$"`. `%N` alone is not enough, since busybox drops
it silently and exits 0, handing back second granularity with no
warning; `$$` differs between concurrent invocations regardless. The
bare-assignment form is kept on purpose -- inlined into an argument, a
failing substitution does not abort under `set -eu` and would yield an
empty constant epoch, restoring the exact false green this prevents.

Pass the same fresh value from script/docker. It is not the CI gate, but
local builds are almost always warm, so it was the likelier fooling in
practice, and two entrypoints disagreeing about whether the tree is
green is worse than either being wrong alone.

The ARG placement from #85 is unchanged, below apk add, COPY go.mod
go.sum and go mod download, so dependency layers still cache and the
build is not cold. Verified by negative control rather than inspection;
measurements are recorded once, in the PR verification comment.

.golangci.yml, the lint-stage FROM line and its digest, script/lint,
REPO_POLICIES.md and .gitea/workflows/check.yml are untouched.

TODO.md

@@ -19,6 +19,42 @@ or delete the branch.
 
 # Completed Steps
 
+- 2026-08-09: Adopted the remaining upstream `CHECK_EPOCH` hardening
+  (issue #91), closing the gap #85 knowingly left open. Four changes,
+  all four decided as adopt upstream in `sneak/prompts` #26. (1) Each
+  check stage now asserts `[ -n "$CHECK_EPOCH" ] || exit 1` before
+  running anything, so a build that supplies no `--build-arg` fails
+  instead of lying. This is the item that mattered: an unset `ARG` is
+  an empty string and an empty string is a stable cache key, so the
+  second and every later bare `docker build .` on an unchanged tree
+  replayed all three check layers and still exited 0 — and `docker
+  build .` is the command `REPO_POLICIES.md` names verbatim as a thing
+  that must be green, so the documented command was precisely the one
+  that lied. Failed steps are never cached, which is what makes the
+  guard fire on every invocation rather than once. (2) The epoch is now
+  expanded into each check command rather than left as a bare
+  declaration, so the cache miss no longer depends on BuildKit's
+  unreferenced-`ARG` handling staying as it is, and the value appears
+  in the build log. (3) `script/cibuild` uses
+  `epoch="$(date +%s%N)$$"`, unique per invocation rather than per
+  second; `%N` alone is insufficient because busybox drops it silently
+  and exits 0, and `$$` is what makes the guarantee hold regardless.
+  The bare-assignment form is kept deliberately — inlined in an
+  argument, a failing substitution does not abort under `set -eu` and
+  would yield an empty constant epoch, restoring the exact false green
+  being fixed. (4) `script/docker` passes the same fresh arg, so the
+  two entrypoints cannot disagree about whether the tree is green;
+  local builds are almost always warm, which made it the likelier
+  fooling in practice. The `ARG` placement from #85 is unchanged, below
+  `apk add`, `COPY go.mod go.sum` and `go mod download`, so dependency
+  layers still cache and the build is not cold. Verified by negative
+  control rather than inspection — a bare `docker build .` run twice
+  back to back, plus back-to-back pairs of both scripts and a host-side
+  `make check`; the measurements are recorded once, in the PR
+  verification comment, rather than restated here. `.golangci.yml`, the
+  lint-stage `FROM` line and its digest, `script/lint`,
+  `REPO_POLICIES.md` and `.gitea/workflows/check.yml` are all
+  untouched.
 - 2026-08-09: Stopped `script/cibuild` from reporting a green it did
   not earn (issue #85). A bare `docker build .` let Docker serve the
   check layers from the layer cache whenever the tree had not changed: