Add PTY-based terminal integration tests

[evansenter]

Problem

The REPL's terminal interaction (reedline integration, raw mode handling, output formatting) is currently untested. This led to a stairstepping bug in PR #102 that wasn't caught until manual testing.

Proposed Solution

Add integration tests using a PTY (pseudo-terminal) library to test actual terminal behavior:

#[test]
fn test_repl_output_no_stairstepping() {
    let mut pty = PtySession::spawn("clemini").unwrap();
    pty.send_line("What is 2+2?").unwrap();
    let output = pty.read_until_prompt().unwrap();
    
    // Each line should start at column 0 (no stairstepping)
    for line in output.lines().skip(1) { // Skip prompt line
        assert!(
            !line.starts_with(' ') || is_intentional_indent(line),
            "Stairstepping detected: {:?}",
            line
        );
    }
}

Libraries to Consider

• expectrl - Rust expect-like library, good for REPL testing
• portable-pty - Cross-platform PTY support
• rexpect - Another expect-style library

Test Cases

1. No stairstepping - Output lines start at column 0
2. Ctrl-C handling - Returns to prompt cleanly
3. History persistence - Commands saved and searchable
4. Multiline input - Shift+Enter creates newlines (may need special key sequence)

Context

• PR feat: add reedline-based REPL with history and improved ctrl-c #102 introduced reedline integration
• Stairstepping bug was caused by raw mode race condition
• Fixed by synchronizing reedline with main loop via ready channel