Fix review findings: stub panics, derivation index, tests, README

- Replace panic() calls in seunlocker_stub.go with error returns,
  following the existing keychainunlocker_stub.go pattern
- Fix hardcoded derivation index 0 in getLongTermKeyForSE: now reads
  vault metadata to use the correct DerivationIndex (matching
  getLongTermPrivateKey in keychainunlocker.go)
- Add tests for SE unlocker exports in secret package (both darwin
  and non-darwin stub tests)
- Update README to reflect SE implementation: remove 'planned' labels,
  update Apple Developer Program references, add secure-enclave to
  unlocker type lists and examples
- Run go fmt on files with import ordering issues

internal/cli/unlockers.go

@@ -1,9 +1,9 @@
 package cli
 
 import (
-	"log"
 	"encoding/json"
 	"fmt"
+	"log"
 	"os"
 	"os/exec"
 	"path/filepath"