Type-check and lint the macOS build from Linux (closes #50)

script/lint-darwin (make lint-darwin; run by script/check, and its
commands by the Dockerfile lint stage) runs go vet and golangci-lint
with GOOS=darwin and cgo off. Compiling cgo for macOS needs Apple's SDK,
so the three functions that call go-keychain, which is cgo there, move
to keychainunlocker_cgo.go; a macOS build without cgo gets
keychainunlocker_nocgo.go and the macse stub, whose errors name the
missing macOS build with cgo. The rest of the keychain unlocker and its
plain-Go tests are now checked; their findings are fixed without
changing behaviour, and lines over 88 columns in the unchecked files
are wrapped.

Model: opus-5-5

script/lint-darwin

@@ -0,0 +1,26 @@
+#!/bin/sh
+# script/lint-darwin: type-check (go vet) and lint the code as a macOS
+# build compiles it, from Linux, in docker only. CI runs on Linux, which
+# never compiles the files built only for macOS. Builds the lint-darwin
+# stage of Dockerfile.lint, rebuilt on every run as script/lint does.
+#
+# Cgo is off: compiling cgo code for macOS needs Apple's SDK headers. That
+# leaves out the files built only with cgo on macOS: the keychain unlocker's
+# calls into the keychain (keychainunlocker_cgo.go, and
+# keychainunlocker_test.go) and the Secure Enclave bindings (internal/macse).
+# Nothing on Linux checks those.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    docker build \
+        --progress=plain \
+        --target lint-darwin \
+        --no-cache-filter=lint-darwin \
+        --output=type=cacheonly \
+        -f Dockerfile.lint .
+}
+
+main "$@"