feat: typewriter text entry

## Summary

Add optional keystroke delay to `shell_send` for typewriter-style text entry during recordings.

## Use Case

When creating demos or recordings, text appearing character-by-character looks more natural and helps viewers follow along.

## Proposal

Add optional `keystroke_delay_ms` parameter to `shell_send`:

```typescript
shell_send({
session_id: "...",
input: "kubectl get pods\r",
keystroke_delay_ms: 100 // sends each character with 100ms delay
})
```

- If omitted or 0: current behavior (entire string sent at once)
- If set: characters sent one at a time with specified delay between each

## Implementation notes

For smooth recordings, `keystroke_delay_ms` should roughly match the frame interval:
- 10 FPS → 100ms delay = ~1 char/frame
- 30 FPS → 33ms+ delay works well

No changes needed to GIF rendering - the existing fixed-FPS capture naturally picks up the intermediate states.