fix(cli): restore SIGPIPE default on Unix (#3657)

Co-authored-by: shanu <shanu@tinyhumans.ai>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/cli_sigpipe.rs

@@ -0,0 +1,50 @@
+#[cfg(unix)]
+#[test]
+fn help_output_closed_pipe_does_not_panic() {
+    use std::io::{BufRead, BufReader};
+    use std::os::unix::process::ExitStatusExt;
+    use std::process::{Command, Stdio};
+
+    let mut child = Command::new(env!("CARGO_BIN_EXE_openhuman-core"))
+        .arg("--help")
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .spawn()
+        .expect("spawn openhuman-core --help");
+
+    // Mirror the issue repro (`openhuman-core --help | head -n 1`): read a
+    // single line, then drop the read end mid-stream so the child's next write
+    // lands on a closed pipe. Closing before reading (as a naive test does)
+    // lets the child buffer its entire few-KB `--help` output in one successful
+    // write well under the 64 KB pipe buffer and exit cleanly, never exercising
+    // the broken-pipe path — a false green that passes even without the fix.
+    let stdout = child.stdout.take().expect("capture stdout");
+    let mut reader = BufReader::new(stdout);
+    let mut first_line = String::new();
+    reader
+        .read_line(&mut first_line)
+        .expect("read first help line");
+    drop(reader);
+
+    let output = child.wait_with_output().expect("wait for openhuman-core");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+
+    assert!(
+        !stderr.contains("Broken pipe"),
+        "stderr must not include a broken-pipe panic: {stderr}"
+    );
+    assert!(
+        !stderr.contains("panicked"),
+        "stderr must not include a panic report: {stderr}"
+    );
+
+    // Acceptance criterion #1: a closed downstream reader must yield a clean
+    // exit — either the child finished before the pipe closed, or the restored
+    // default disposition let SIGPIPE terminate it. A normal-code crash (e.g.
+    // the panic exit code 101) is precisely the regression this guards against.
+    assert!(
+        output.status.success() || output.status.signal() == Some(libc::SIGPIPE),
+        "process must exit cleanly or via SIGPIPE, got {:?}",
+        output.status
+    );
+}