refactor(command): improve formula commitment and buffer management

Refactor command execution and buffer management.

- `CommitFormula` now defaults to targeting `_Measure` .
- `CommitFormula` no longer automatically clears the buffer; buffer
  clearing is now handled by keymap sequences.
- Added `ClearBufferCmd` to the command registry.
- Updated `AddFormulaCmd` to support optional target category.
- Added `SetBuffer` effect to allow clearing buffers.

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

src/ui/effect.rs

@@ -1252,6 +1252,23 @@ mod tests {
         assert_eq!(app.mode, AppMode::Help);
     }
 
+    /// SetBuffer with empty value clears the buffer (used by clear-buffer command
+    /// in keymap sequences after commit).
+    #[test]
+    fn set_buffer_empty_clears() {
+        let mut app = test_app();
+        app.buffers.insert("formula".to_string(), "old text".to_string());
+        SetBuffer {
+            name: "formula".to_string(),
+            value: String::new(),
+        }
+        .apply(&mut app);
+        assert_eq!(
+            app.buffers.get("formula").map(|s| s.as_str()),
+            Some(""),
+        );
+    }
+
     #[test]
     fn set_status_effect() {
         let mut app = test_app();