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

README.md

@@ -519,10 +519,15 @@ them. We provide:
 - `script/lint` — run `golangci-lint` in docker only: builds
   `Dockerfile.lint`, where the linter is a build step that runs on every
   call, also on an unchanged tree
+- `script/lint-darwin` — run `go vet` and `golangci-lint` in docker on
+  the code as a macOS build compiles it (`GOOS=darwin`), which a Linux
+  build never compiles; cgo is off, so the keychain unlocker
+  (`internal/secret/keychainunlocker.go` and its tests) and the Secure
+  Enclave bindings (`internal/macse`) are not checked
 - `script/fmt` — format all Go code (writes)
 - `script/fmt-check` — check formatting without writing
-- `script/check` — run `script/test`, `script/lint`, and
-  `script/fmt-check`
+- `script/check` — run `script/test`, `script/lint`,
+  `script/lint-darwin`, and `script/fmt-check`
 - `script/docker` — build the Docker image tagged with the project name
 - `script/cibuild` — CI entrypoint: `docker build --ulimit
   memlock=-1:-1 .` (memguard needs mlock; the Dockerfile runs the