chore: finish the vocabulary sweep — KV_ERROR, not REDIS_ERROR

Last of our own naming to move. The error-code fixture in the wrapping test (and
the README example that mirrors it) still said REDIS_ERROR.

What intentionally remains, and why:
  RediSearch  — names a specific external module that the FT.* commands map to.
                It is an address, not our vocabulary.
  StoreDist   — matches a case-insensitive search for "redis" by accident
                (sto-reDis-t). Not the word.
  RELEASE-NOTES.md — a changelog of what upstream shipped. Rewriting history to
                pretend it used our names would make it wrong.

README.md

@@ -477,7 +477,7 @@ func (h MyHook) ProcessHook(next kv.ProcessHook) kv.ProcessHook {
         if err != nil {
             // Wrap with custom error type
             wrappedErr := &AppError{
-                Code:      "REDIS_ERROR",
+                Code:      "KV_ERROR",
                 RequestID: getRequestID(ctx),
                 Err:       err,
             }

error_wrapping_test.go

@@ -384,7 +384,7 @@ func TestCustomErrorTypeWrapping(t *testing.T) {
 	cmd1 := kv.NewStatusCmd(testCtx, "GET", "key")
 	loadingErr := proto.ParseErrorReply([]byte("-LOADING KV is loading the dataset in memory"))
 	customErr1 := &AppError{
-		Code:      "REDIS_ERROR",
+		Code:      "KV_ERROR",
 		Message:   "Database operation failed",
 		RequestID: "req-12345",
 		Err:       loadingErr,
@@ -398,7 +398,7 @@ func TestCustomErrorTypeWrapping(t *testing.T) {
 
 	// Verify error message contains custom context
 	errMsg := cmd1.Err().Error()
-	if !contains(errMsg, "REDIS_ERROR") || !contains(errMsg, "req-12345") {
+	if !contains(errMsg, "KV_ERROR") || !contains(errMsg, "req-12345") {
 		t.Errorf("Error message missing custom error context: %v", errMsg)
 	}