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/command/cmd/effect_cmds.rs

@@ -200,11 +200,35 @@ effect_cmd!(
 effect_cmd!(
     AddFormulaCmd,
     "add-formula",
-    |args: &[String]| require_args("add-formula", args, 2),
+    |args: &[String]| {
+        if args.is_empty() || args.len() > 2 {
+            return Err(format!("add-formula requires 1-2 argument(s), got {}", args.len()));
+        }
+        Ok(())
+    },
     |args: &Vec<String>, _ctx: &CmdContext| -> Vec<Box<dyn Effect>> {
+        // 1 arg: formula text (target_category defaults to _Measure)
+        // 2 args: target_category, formula text
+        let (cat, raw) = if args.len() == 2 {
+            (args[0].clone(), args[1].clone())
+        } else {
+            ("_Measure".to_string(), args[0].clone())
+        };
         vec![Box::new(effect::AddFormula {
-            target_category: args[0].clone(),
-            raw: args[1].clone(),
+            target_category: cat,
+            raw,
+        })]
+    }
+);
+
+effect_cmd!(
+    ClearBufferCmd,
+    "clear-buffer",
+    |args: &[String]| require_args("clear-buffer", args, 1),
+    |args: &Vec<String>, _ctx: &CmdContext| -> Vec<Box<dyn Effect>> {
+        vec![Box::new(effect::SetBuffer {
+            name: args[0].clone(),
+            value: String::new(),
         })]
     }
 );