Support JSON output for list, search, and agenda

[Aurelian-Shuttleworth]

Problem

Current output formats (ASCII tables, CSV) are designed for human consumption. Integrating gcalcli into dashboards (Polybar, Conky) or observability pipelines requires fragile regex parsing of whitespace-aligned text, which breaks easily when column widths change or data contains unexpected characters.

Solution

Implement structured JSON output for data retrieval commands (list, search, agenda).

• Schema: Output a list of objects with strongly typed fields (ISO 8601 dates, full descriptions).
• Parsability: Enable use of standard tools like jq for filtering and transformation.
• Observability: Facilitate easier ingestion of calendar data into monitoring dashboards or custom status bars without fragile text scraping.