Filename templates now support both {{variable}} (preferred) and
{variable} (legacy) syntax for backwards compatibility. This makes
the syntax consistent with body/frontmatter templates.
Changes:
- filenameGenerator.ts: Process {{var}} first, then {var} as fallback
- titlePropertyCard.ts: Add deprecation warning for single-brace syntax
- en.ts: Add translation for legacy syntax warning
- docs/task-defaults.md: Update examples to use {{variable}} syntax
The single-brace syntax remains functional but users will see a warning
in settings encouraging migration to double-brace syntax.