feat: record existing terminal session

## Summary

Allow shellwright to attach to and record an existing terminal session, rather than spawning a new one.

## Use Case

Currently shellwright spawns processes via `shell_start`. But often you're already running something (claude code, vim, htop) and want to start recording mid-session. This feature would let you attach shellwright to an existing PTY and capture what's happening.

## Proposed Approach

New tool: `shell_attach` or `shell_record_external`

Options:
1. **Attach by PID** - attach to an existing process's PTY
2. **Attach by TTY** - attach to a specific TTY device (e.g., `/dev/ttys001`)
3. **Pipe mode** - shellwright reads from stdin, useful with `script` or `tmux`

## Example Usage

```json
{
"tty": "/dev/ttys001",
"cols": 80,
"rows": 24
}
```

Or integration with `tmux capture-pane` / `script` command.

## Technical Considerations

- Reading from existing PTY requires appropriate permissions
- May need to poll/capture the terminal buffer periodically
- Could leverage `tmux` or `screen` for easier capture
- Alternative: provide a CLI mode where shellwright wraps an existing command

## Alternative: Wrapper Mode

```bash
shellwright wrap -- claude
```

This would be simpler - shellwright spawns the process but the user interacts with it directly in their terminal, while shellwright records in the background.