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

TODO.md

@@ -25,6 +25,27 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-04: `script/lint-darwin` (`make lint-darwin`) runs `go vet` and
+  `golangci-lint` in docker on the code as a macOS build compiles it
+  (`GOOS=darwin`), with cgo off
+  (https://git.eeqj.de/sneak/secret/issues/50). `script/check` runs it, and
+  the `Dockerfile` lint stage runs its commands, so `script/cibuild` does too.
+  Before, CI on Linux never compiled the files built only for macOS. Compiling
+  cgo code for macOS needs Apple's SDK headers, and both `internal/macse` and
+  `github.com/keybase/go-keychain` are cgo on macOS. So the three functions
+  that call `go-keychain` moved from `keychainunlocker.go` to
+  `keychainunlocker_cgo.go`, built only with cgo on macOS like
+  `macse_darwin.go`. A macOS build without cgo, which before did not compile,
+  gets `keychainunlocker_nocgo.go` and the `macse` stub instead, whose errors
+  say the keychain or Secure Enclave needs a macOS build with cgo. The check
+  covers the rest of the keychain unlocker, the Secure Enclave unlocker and
+  the macOS-only tests other than `keychainunlocker_test.go`, whose lint
+  findings are fixed. For the length and complexity limits, parts of
+  `GetIdentity`, `getLongTermPrivateKey` and `CreateKeychainUnlocker` moved
+  into functions of their own, and the Secure Enclave unlocker derives the
+  long-term key from the mnemonic through the same function as the keychain
+  unlocker instead of a copy of it. Lines over 88 columns in the files the
+  check cannot see are wrapped.
 - 2026-10-04: `secret rm`, `secret version rm`, `secret vault remove` and
   `secret unlocker remove` ask `[y/N]` before removing anything
   (https://git.eeqj.de/sneak/secret/issues/39), naming what they remove: the
@@ -297,11 +318,14 @@ Bring the repo into policy compliance in one commit:
 - Cover mnemonic-vs-xprv identity consistency in
   `pkg/agehd/agehd_test.go` `TestMnemonicVsXPRVConsistency` (was an
   in-code FIXME removed for godox).
-- Darwin-gated files (`internal/secret/keychainunlocker.go`,
-  `seunlocker_darwin.go`, `internal/macse/macse_darwin.go`, related
-  tests) are not linted on the Linux CI runner and still contain lines
-  over the new 88-column limit; they will surface if lint ever runs on
-  macOS.
+- CI does not compile, lint or test the files built only with cgo on
+  macOS, since compiling them needs Apple's SDK:
+  `internal/secret/keychainunlocker_cgo.go` (the three functions that call
+  `go-keychain`) with `keychainunlocker_test.go`, and `internal/macse`
+  (`macse_darwin.go`, `macse_test.go`, the Objective-C sources). Lint has
+  never run on them, so it would likely find more there than the line
+  lengths. No macOS test runs in CI. A macOS runner would cover all of it
+  (asked on https://git.eeqj.de/sneak/secret/issues/50).
 - Merge secure-enclave-unlocker to main once review is done.
 - 1.0 critical security blockers (from repo TODO.md):
     - Command injection: GPG key IDs passed unescaped to exec.Command