Add SSH readiness wait for AWS auto-start in shell_connect

## Description

When `shell_connect()` auto-starts a stopped AWS instance, it calls `aws_start()` which waits for the instance to be running and gets the new IP. However, there's a potential timing issue:

1. Instance starts and gets a new public IP
2. `save_known_host` updates the known_hosts file
3. SSH connection is attempted immediately

The SSH daemon on the instance may not be ready yet even though the instance state is "running". The SSM path already has a wait loop for the SSM agent, but the direct SSH path doesn't wait for port 22 to be available.

## Recommended fix

Add a `wait_for_ssh()` helper that polls port 22 with a timeout before attempting the SSH connection.

## Priority

Low — users can just retry the connection.