feat: standardize date format to ISO 8601 (YYYY-MM-DD)

[scarf005]

Summary

This PR standardizes date formatting across the codebase to use the ISO 8601 format (YYYY-MM-DD) instead of locale-dependent formats.

Changes

• Updated Locale.datetime() in packages/opencode/src/util/locale.ts to use en-CA locale for toLocaleDateString(), which outputs dates in YYYY-MM-DD format
• Renamed variable from localDate to isoDate for clarity

Rationale

• Consistency: ISO 8601 is an internationally recognized standard for date representation
• Sortability: YYYY-MM-DD format is lexicographically sortable
• Clarity: Removes ambiguity between MM/DD/YYYY (US) and DD/MM/YYYY (European) formats
• Interoperability: Better for data exchange and API responses