feat: implement --new flag

The new `--new` or `-n` flag allows to create a new revision after the
single revision being described.

Running `jj-cz --new` is the equivalent of running `jj-cz --new @`,
describing the current revision and creating a new revision after it.

Passing several revisions to `jj-cz` with the `--new` flag will result
in an error.

src/prompts/workflow.rs

@@ -184,6 +184,10 @@ impl<J: JjExecutor, P: Prompter> CommitWorkflow<J, P> {
             Err(Error::Cancelled)
         }
     }
+
+    pub async fn new_revision(&self, revset: &str) -> Result<(), Error> {
+        self.executor.new_revision(revset).await
+    }
 }
 
 #[cfg(test)]
@@ -762,4 +766,59 @@ mod tests {
         assert_eq!(messages.len(), 1);
         assert_eq!(messages[0], "fix: fix crash");
     }
+
+    /// Test workflow new_revision() records the revset
+    #[tokio::test]
+    async fn workflow_new_revision_records_revset() {
+        let mock_executor = MockJjExecutor::new();
+        let workflow = CommitWorkflow::new(mock_executor);
+
+        let result = workflow.new_revision("@").await;
+        assert!(result.is_ok());
+
+        let calls = workflow.executor.new_revision_calls();
+        assert_eq!(calls, vec!["@"]);
+    }
+
+    /// Test workflow new_revision() propagates executor errors
+    #[tokio::test]
+    async fn workflow_new_revision_propagates_error() {
+        let mock_executor =
+            MockJjExecutor::new().with_new_revision_response(Err(Error::RepositoryLocked));
+        let workflow = CommitWorkflow::new(mock_executor);
+
+        let result = workflow.new_revision("@").await;
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), Error::RepositoryLocked));
+    }
+
+    /// Test workflow run_for_revset() followed by new_revision() records both
+    ///
+    /// This mirrors the actual usage pattern in main.rs.
+    #[tokio::test]
+    async fn workflow_describe_then_new_revision() {
+        let mock_executor = MockJjExecutor::new().with_is_repo_response(Ok(true));
+        let mock_prompts = MockPrompts::new()
+            .with_commit_type(CommitType::Feat)
+            .with_scope(Scope::empty())
+            .with_description(Description::parse("add feature").unwrap())
+            .with_breaking_change(BreakingChange::No)
+            .with_body(Body::default())
+            .with_confirm(true);
+
+        let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
+
+        workflow.run_for_revset("@").await.expect("describe failed");
+        workflow
+            .new_revision("@")
+            .await
+            .expect("new_revision failed");
+
+        let messages = workflow.executor.describe_messages();
+        assert_eq!(messages.len(), 1);
+        assert!(messages[0].contains("feat:"));
+
+        let calls = workflow.executor.new_revision_calls();
+        assert_eq!(calls, vec!["@"]);
+    }
 }