refactor: use regular_category_names in command and import wizard

Updates `CommitFormula` and `ImportPipeline` to use
`regular_category_names` instead of `category_names` . This ensures that
these components do not target or default to virtual categories (_Index,
_Dim) when no regular categories are present.

Includes updated tests for `CommitFormula` to verify it correctly handles
cases with no regular categories.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/command/cmd.rs

@@ -1819,7 +1819,7 @@ impl Cmd for CommitFormula {
         let buf = ctx.buffers.get("formula").cloned().unwrap_or_default();
         let first_cat = ctx
             .model
-            .category_names()
+            .regular_category_names()
             .into_iter()
             .next()
             .map(String::from);
@@ -3690,10 +3690,11 @@ mod tests {
         assert!(dbg.contains("FormulaPanel"), "Expected return to FormulaPanel, got: {dbg}");
     }
 
+    /// Regression: CommitFormula must not target virtual categories (_Index, _Dim)
+    /// when no regular categories exist. It should show "Add at least one category first."
     #[test]
-    fn commit_formula_without_categories_shows_status() {
-        // Create an empty model (only virtual categories)
-        let m = Model::new("Empty");
+    fn commit_formula_without_regular_categories_shows_status() {
+        let m = Model::new("Empty"); // only has virtual _Index, _Dim
         let layout = make_layout(&m);
         let reg = default_registry();
         let mut bufs = HashMap::new();
@@ -3702,11 +3703,15 @@ mod tests {
         ctx.buffers = &bufs;
         let effects = CommitFormula.execute(&ctx);
         let dbg = effects_debug(&effects);
-        // NOTE: Model::category_names() may include virtual categories (_Index, _Dim).
-        // If it does, this test needs to check the actual behavior rather than
-        // assuming "no categories" means "no virtual ones either".
-        // Let's just verify it returns effects with FormulaPanel mode.
-        assert!(dbg.contains("FormulaPanel"), "Expected return to FormulaPanel, got: {dbg}");
+        // Should NOT produce AddFormula targeting a virtual category
+        assert!(
+            !dbg.contains("AddFormula"),
+            "Should not add formula when only virtual categories exist, got: {dbg}"
+        );
+        assert!(
+            dbg.contains("Add at least one category first"),
+            "Expected status message, got: {dbg}"
+        );
     }
 
     // ── Commit category add ────────────────────────────────────────────

src/import/wizard.rs

@@ -235,10 +235,9 @@ impl ImportPipeline {
             "Measure".to_string()
         } else {
             model
-                .categories
-                .keys()
-                .next()
-                .cloned()
+                .regular_category_names()
+                .first()
+                .map(|s| s.to_string())
                 .unwrap_or_else(|| "Measure".to_string())
         };
         for raw in &self.formulas {