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,
and keychainunlocker.go uses go-keychain, which is cgo there, so it and
its tests are now built only with cgo on macOS, like internal/macse;
their stubs serve a macOS build without cgo. checkMacOSAvailable moves
to seunlocker_darwin.go. The findings in the newly checked files are
fixed, and lines over 88 columns in the unchecked ones are wrapped.

Model: opus-5-5

Dockerfile.lint

@@ -1,6 +1,6 @@
-# Lint image, built by script/lint: golangci-lint runs as a build step, so a
-# successful build is a clean lint. Works where the docker daemon is remote
-# and bind mounts are impossible.
+# Lint image, built by script/lint and script/lint-darwin: golangci-lint runs
+# as a build step, so a successful build is a clean lint. Works where the
+# docker daemon is remote and bind mounts are impossible.
 
 # golangci/golangci-lint:v2.12.2 (Debian-based), 2026-08-07
 FROM golangci/golangci-lint:v2.12.2@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS deps
@@ -17,3 +17,13 @@ FROM deps AS lint
 COPY . .
 
 RUN golangci-lint run --config .golangci.yml ./...
+
+# script/lint-darwin rebuilds this stage on every run, by this name. It
+# checks the code as a macOS build compiles it, but with cgo off, which
+# leaves out the files that need cgo on macOS (see script/lint-darwin).
+FROM deps AS lint-darwin
+
+COPY . .
+
+RUN GOOS=darwin CGO_ENABLED=0 go vet ./...
+RUN GOOS=darwin CGO_ENABLED=0 golangci-lint run --config .golangci.yml ./...