Change but branch new to output only branch name for agent use

Modified the output of `but branch new` command to print just the branch name
instead of "Created branch {branch_name}". This makes it easier for agents and
scripts to capture and use the output directly with other commands like
`but commit --branch=[identifier]` or `but push [identifier]`.

Added tests to verify the new output format.

Co-authored-by: Byron <63622+Byron@users.noreply.github.com>

Author: Copilot

crates/but/src/branch/mod.rs

@@ -126,7 +126,7 @@ pub async fn handle(
                     anchor,
                 },
             )?;
-            println!("Created branch {branch_name}");
+            println!("{branch_name}");
             Ok(())
         }
         Some(Subcommands::Delete { branch_name, force }) => {

crates/but/tests/but/branch.rs

@@ -0,0 +1,51 @@
+use crate::utils::{Sandbox, setup_metadata};
+
+#[test]
+fn branch_new_outputs_branch_name() -> anyhow::Result<()> {
+    let env = Sandbox::init_scenario_with_target("one-stack")?;
+    insta::assert_snapshot!(env.git_log()?, @r"
+    * edd3eb7 (HEAD -> gitbutler/workspace) GitButler Workspace Commit
+    * 9477ae7 (A) add A
+    * 0dc3733 (origin/main, origin/HEAD, main) add M
+    ");
+
+    setup_metadata(&env, &["A"])?;
+
+    // Create a new branch and capture the output
+    let output = env
+        .but("branch new my-feature")
+        .assert()
+        .success()
+        .get_output()
+        .stdout
+        .clone();
+
+    let output_str = String::from_utf8(output)?;
+
+    // The output should be just the branch name (with newline)
+    assert_eq!(output_str.trim(), "my-feature");
+
+    Ok(())
+}
+
+#[test]
+fn branch_new_with_anchor() -> anyhow::Result<()> {
+    let env = Sandbox::init_scenario_with_target("one-stack")?;
+    setup_metadata(&env, &["A"])?;
+
+    // Create a new branch with an anchor (using longer ID)
+    let output = env
+        .but("branch new my-feature --anchor 9477ae7")
+        .assert()
+        .success()
+        .get_output()
+        .stdout
+        .clone();
+
+    let output_str = String::from_utf8(output)?;
+
+    // The output should be just the branch name (with newline)
+    assert_eq!(output_str.trim(), "my-feature");
+
+    Ok(())
+}

crates/but/tests/but/main.rs

@@ -1,3 +1,4 @@
+mod branch;
 mod commit;
 mod journey;
 mod rub;