fix: allow custom variable references in datetime helper attributes

[fuleinist]

Summary

The datetime helper's delta_days (and similar time-based attributes) can now reference custom variable names defined in the template frontmatter. For example:

template_todo_alarm: '{{datetime delta_days=days_until_due set_time="09:00"}}'

This fixes issue #161 where the README example was broken.

Problem

The README shows this example for setting a todo alarm:

template_todo_alarm: {{datetime delta_days=days_until_due set_time="09:00" format="YYYY-MM-DD HH:mm"}}

However, this was broken because when the datetime helper is called within the template compilation context, Handlebars passes the literal string "days_until_due" to the helper instead of resolving it to the actual variable value.

Solution

1. src/helpers/datetime.ts: Added variable resolution logic that looks up context variable values from Handlebars 'this' and 'options.data.root'. When an attribute value is a string that matches a context variable name, it resolves to that variable's actual value.

2. src/parser.ts: Updated to include processed variable values (numbers, dates, etc.) in the Handlebars context when compiling special variable templates. This ensures the datetime helper can resolve variable names to their actual values.

Testing

• Added unit test that reproduces issue readme: template_todo_alarm example is broken #161: uses a custom number variable 'days_until_due' with the datetime helper in template_todo_alarm
• All 99 tests pass

Fixes #161