Shell timeout doesn't cover stdout/stderr reads; sequential reads can deadlock

[tcdent]

Bug

In src/tools/io.rs, execute_shell() reads stdout and stderr sequentially before applying the timeout to wait():

// Read ALL of stdout (blocks until EOF)
if let Some(stdout) = stdout { /* read all lines */ }
// Read ALL of stderr (blocks until EOF)  
if let Some(stderr) = stderr { /* read all lines */ }

// THEN apply timeout to wait()
let status = tokio::time::timeout(timeout_secs, child.0.wait()).await;

Problems

1. Timeout doesn't cover I/O reads — If a command holds stdout open (e.g. tail -f, a hung process), reader.next_line().await blocks forever. The tokio::time::timeout is never reached.

2. Sequential reads can deadlock — If a process fills the stderr pipe buffer (~64KB) while the parent is still reading stdout, the child blocks on stderr write, the parent blocks on stdout read waiting for EOF → deadlock.

Fix

Wrap stdout, stderr reads and wait in a single tokio::join! under one timeout:

let result = tokio::time::timeout(
    Duration::from_secs(timeout_secs),
    async {
        let stdout_fut = async { /* read all stdout */ };
        let stderr_fut = async { /* read all stderr */ };
        let wait_fut = child.0.wait();
        tokio::join!(stdout_fut, stderr_fut, wait_fut)
    }
).await;

Found during code review of the cancellation PR (#61).