Remove the keychain and Secure Enclave unlockers (closes #132)

Neither unlocker works without macOS code signing and entitlements, which
secret does not have, so both were research and leave the build.
`secret unlocker add` takes only passphrase and pgp on every platform;
keychain and secure-enclave fail as unknown types and are gone from
--help and shell completion. internal/macse, their files in
internal/secret with their tests, and their branches in internal/vault
and internal/cli are deleted; github.com/keybase/go-keychain leaves
go.mod and go.sum. README and Dockerfile comments no longer mention
them. The research stays at ef0ae90, tagged v1.0.0-rc1.

Judgement call: derivation_index_test.go goes with getLongTermPrivateKey,
its only subject, which only these two unlockers used.
Unverified: not built or run on a Mac; only the GOOS=darwin vet and lint ran.

Model: opus-5-5

go.mod

@@ -12,7 +12,6 @@ require (
 	github.com/creack/pty v1.1.25-0.20260601142114-9246436fffe8 // v1.1.24's Open can return another pty's terminal
 	github.com/dustin/go-humanize v1.0.1
 	github.com/fatih/color v1.18.0
-	github.com/keybase/go-keychain v0.0.0-20230307172405-3e4884637dd1
 	github.com/oklog/ulid/v2 v2.1.1
 	github.com/spf13/afero v1.14.0
 	github.com/spf13/cobra v1.9.1