feat: Support standard YAML list syntax for 'triggers' input

The current bash-style array syntax triggers: ('path/') is... a product of its time. It’s prone to syntax errors and feels non-standard compared to other GitHub Actions.

Proposal: Support a standard YAML list (or a multiline string) for the triggers input.

Example:

```yaml
triggers:
- backend/
- frontend/
```

Exit Strategy: The action should check the input format. If it detects a YAML-style list, it handles it natively. If it sees the old ('path/') format, it continues to support it (with a deprecation warning) until the next major version. This gives us a clean path to 2026-standard YAML.