CLI: a closed stdout ends vectorcraft-cli quietly

`vectorcraft-cli commands | head -1` panicked and exited with status 101:

  thread 'main' panicked at library/std/src/io/stdio.rs:1168:9:
  failed printing to stdout: Broken pipe (os error 32)

println! and print! panic when stdout is closed. Output now goes through
outln! and out!, which end the program with status 0 on a broken pipe (the
reader has what it wanted) and report any other write error on stderr with
status 1.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/vectorcraft-cli/tests/cli.rs

@@ -82,3 +82,17 @@ fn mcp_headless_over_stdio() {
     assert_eq!(replies[2]["result"]["isError"], false);
     assert_eq!(replies[3]["result"]["content"][0]["type"], "image");
 }
+
+/// `vectorcraft-cli commands | head -1` panicked with "failed printing to
+/// stdout: Broken pipe (os error 32)" and exit status 101.
+#[test]
+fn closed_stdout_ends_quietly() {
+    use std::io::Read;
+    let mut child = Command::new(BIN).arg("commands").stdout(Stdio::piped()).stderr(Stdio::piped()).spawn().unwrap();
+    let mut first = [0u8; 16];
+    child.stdout.as_mut().unwrap().read_exact(&mut first).unwrap();
+    drop(child.stdout.take());
+    let out = child.wait_with_output().unwrap();
+    let err = String::from_utf8_lossy(&out.stderr);
+    assert!(out.status.success() && !err.contains("panicked"), "{:?}: {err}", out.status);
+}