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

internal/secret/pgpunlocker.go

@@ -171,7 +171,6 @@ func (p *PGPUnlocker) GetID() string {
 // Remove implements Unlocker interface - removes the PGP unlocker
 func (p *PGPUnlocker) Remove() error {
 	// For PGP unlockers, we just need to remove the directory
-	// No external resources (like keychain items) to clean up
 	err := RemoveDirAtomic(p.fs, p.Directory)
 	if err != nil {
 		return fmt.Errorf("failed to remove PGP unlocker directory: %w", err)