feat(openshell-vm): add tty support for exec

[benoitf]

Summary

Add TTY support for openshell-vm exec, enabling interactive terminal sessions inside VMs.
Both the Rust host (client-side raw mode, terminal resize propagation) and the Python guest agent (PTY allocation, resize handling) are updated to support the new tty flag

openshell-exec-tty.mov

Related Issue

#936

Changes

• Rust host (crates/openshell-vm/src/exec.rs):
  • Add Resize { cols, rows } variant to ClientFrame for terminal resize events
  • Add RawModeGuard RAII struct that switches stdin to raw mode and restores on drop
  • Add get_terminal_size() helper using TIOCGWINSZ ioctl
  • Send initial terminal size before starting the stdin pump when tty is enabled
  • Detect terminal resize changes during pump_stdin and forward Resize frames
  • Add comprehensive unit tests for frame serialization, deserialization, round-trip compatibility, env validation, and Python agent wire-format compatibility
• Python guest agent (crates/openshell-vm/scripts/openshell-vm-exec-agent.py):
  • Add set_winsize() helper to apply terminal dimensions via TIOCSWINSZ ioctl
  • Add handle_client_tty() handler that allocates a PTY (pty.openpty), attaches the subprocess to it, and relays I/O over the VSOCK protocol
  • Refactor handle_client() to dispatch between pipe mode (handle_client_pipe) and TTY mode based on the tty field in the exec request
  • Handle resize frames to dynamically update PTY dimensions

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)