Fail the Docker build when the lint stage's linter is not the pin

The reordered COPY --from=lint did not make the two stages provably one
toolchain, as the Dockerfile comment, the previous commit message and
TODO.md all claimed. script/bootstrap compares its pin against whatever
PATH resolves, and $GOPATH/bin sits ahead of /usr/local/bin, so any
drift was absorbed: bootstrap rebuilt the pinned version from source,
verified that, and the build went green with the lint stage having
linted at one version and make check having run at another. Bumping the
lint stage image without touching the pin was enough to produce it.

New script/verify-linter-pin fails, naming both versions, unless a given
golangci-lint binary is exactly the version script/bootstrap pins. The
build stage runs it on the binary copied out of the lint stage,
immediately after the copy and before bootstrap, so no reinstall can
satisfy it. The pin is read out of script/bootstrap, which stays its
single source of truth; a pin that cannot be read is a hard failure
rather than a skip. The check takes no CHECK_EPOCH because its only
inputs are the copied binary and script/, so Docker invalidates the
layer exactly when a cached result would stop being true.

The linter version is pinned independently in the lint stage's image
digest and in GOLANGCI_LINT_VERSION, with nothing keeping them in sync;
a half-applied bump is now a build failure instead of a silent split.

ENV PATH keeps $GOPATH/bin, but its comment no longer claims a reinstall
is the reason: bootstrap must be able to run and verify what it
installs, and nothing in this image is shadowed by the entry.

Verified: with the lint stage's linter faked to 2.11.0 after the gates
had really run, the build fails at verify-linter-pin naming 2.11.0 and
2.12.2, with bootstrap and the check gate never reached; an unmodified
make docker is green with all three gates run on a fresh epoch and real
test results. A planted unused finding still fails at the lint stage
with gate check absent from the log; the image still fails
TestScanHardlinkRunFailsTogether under --user 0:0 and passes as uid
1000, both with the Go test cache disabled; and a second build serves
bootstrap, the verify layer and the dependency layers CACHED while the
gates go cold.

README.md

@@ -493,6 +493,16 @@ and may be invoked directly. The provided entrypoints are:
 -   `script/install-precommit` — install the git pre-commit hook that
     runs `script/precommit`. The hook is written to the common git
     directory, so the main checkout and every worktree share it.
+-   `script/verify-linter-pin` — fail unless a `golangci-lint` binary
+    (given as its argument, default whatever `PATH` resolves) is
+    exactly the version `script/bootstrap` pins, naming both versions
+    if not. The `Dockerfile` build stage runs it on the linter it
+    copies out of the lint stage: the version is pinned independently
+    in the lint stage's image digest and in `script/bootstrap`, and
+    bumping one alone would otherwise be absorbed silently by
+    bootstrap rebuilding its pin from source, leaving the two stages
+    on different linters under a green build. The pin is read from
+    `script/bootstrap`, which stays its single source of truth.
 
 `script/docker` and `script/cibuild` both pass a freshly computed
 `CHECK_EPOCH` build argument, and the `Dockerfile`'s gate steps