feat(store): envelope DEK with admin/agent wrap slots

Open() now opens LOCKED; InitKeys generates a DEK sealed under both KEKs;
Unlock loads it from the role's slot (admin slot has no agent fallback).
s.key becomes the DEK, so account/mail crypto is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/store/store.go

@@ -19,7 +19,9 @@ type Store struct {
 }
 
 // Open opens (creating if needed) the DB at path and applies the schema.
-func Open(path string, key []byte) (*Store, error) {
+// The store opens LOCKED: call InitKeys (first run) or Unlock before any
+// secret read/write.
+func Open(path string) (*Store, error) {
 	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
 		return nil, fmt.Errorf("create db dir: %w", err)
 	}
@@ -39,7 +41,7 @@ func Open(path string, key []byte) (*Store, error) {
 		db.Close()
 		return nil, fmt.Errorf("apply schema: %w", err)
 	}
-	s := &Store{db: db, key: key}
+	s := &Store{db: db}
 	if _, err := s.GetSetting("schema_version"); err != nil {
 		if err := s.SetSetting("schema_version", strconv.Itoa(schemaVersion)); err != nil {
 			db.Close()