cli countdown support

Though countdowns are exposed through the TUI, I can't find that information in the CLI:

```sh

# 5 events today
❯ hey event day --count
5

# no 'countdown' attributes in the json
❯ hey event day --json | grep countdown

# there is a reminders key, but that's different in hey to countdowns
❯ hey event day --json | grep reminder
"reminders": [
"reminders_label": "30 minutes before",
"reminders": [
"reminders_label": "30 minutes before",
"reminders": [
"reminders_label": "30 minutes before",
"reminders": [
"reminders_label": "30 minutes before",
"reminders": [
"reminders_label": "30 minutes before",
"reminders": [
"reminders_label": "30 minutes before",
"reminders": [
"reminders_label": "30 minutes before",
"reminders": [
"reminders_label": "30 minutes before",
```

The hey cli is a terrific addition to my organisation, even without this, thanks so much folks!

Start by comparing the TUI's countdown handling with the CLI implementation of `hey event day`, especially its `--json` output. Confirm the expected countdown fields from the TUI, then make the CLI expose them and verify that `hey event day --json` includes countdown attributes without confusing them with reminders.

Written by the indexing model from the issue text.