feat: Aggregation functions

[daavidrgz]

Summary

Add built-in aggregation functions for common data analysis operations on arrays and collections.

Proposed Functions

Function	Description	Example
count	Count elements	{ users { count } }
sum	Sum numeric values	{ orders { sum(total) } }
avg	Average of numeric values	{ scores { avg(value) } }
min / max	Min/max value	{ prices { min(amount) } }
sort_by	Sort elements by field	{ users { sort_by(age) } }
group_by	Group elements by field	{ logs { group_by(level) } }
unique	Deduplicate values	{ tags { unique } }
flatten	Flatten nested arrays	{ nested_lists { flatten } }

Example Usage

{
  orders(status = "completed") {
    count,
    sum(total),
    avg(total)
  }
}

Motivation

These are what people actually need when exploring data from APIs or log files. Without aggregation, users still have to reach for jq or Python for anything analytical. Adding these makes gq a complete data exploration tool from the CLI.

Considerations

• Should aggregations be functions or special query syntax?
• How do aggregations interact with nested queries?
• sort_by and group_by need to support ascending/descending and multiple keys
• Type handling: what happens when sum encounters non-numeric values?