Query aggregators

[danielrothmann]

This PR adds support for aggregation clauses to queries, allowing you to compute summary values from an entity result set.

Previously, queries always returned a bag of entities. To get high-level insights (e.g. count of open tasks, total invoice amount), you'd need to export and process the data externally.

Aggregations go at the end of a query as a final clause. They transform the entity set into a single value or extracted fields. The implementation adds an Aggregation enum to the query data model, a new AggregationResult return type, per-aggregation execution modules, and grammar/parser support for parsing them.

Supported aggregations:

• select <field>, ... — extract specific field values
• count [<field>] — count entities (optionally by field presence)
• sum <field> — sum a numeric field (integer, float, or currency)
• average <field> — compute the mean
• median <field> — compute the median

Syntax:

from task | where is_completed == false | count
from invoice | where status == "sent" | sum amount
from task | select @id, name, due_date

Closes #37