feat: standardize date format to ISO 8601

[scarf005]

Problem

The codebase uses locale-dependent date formats which causes:

• Inconsistency across different regions
• Ambiguity between MM/DD/YYYY (US) and DD/MM/YYYY (European)
• Non-sortable date strings
• Interoperability issues

Solution

Standardize to ISO 8601 format (YYYY-MM-DD):

• Update Locale.datetime() to use en-CA locale
• Ensures consistent YYYY-MM-DD output

Benefits

• Consistency: International standard
• Sortability: Lexicographically sortable
• Clarity: No ambiguity
• Interoperability: Better for APIs and data exchange

Related: Reopening #6450

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• feat: standardize date format to ISO 8601 (YYYY-MM-DD) #6450: standardize date format to ISO 8601 (YYYY-MM-DD) - Previously closed, this appears to be reopening or re-addressing that work
• Session timestamps display in UTC instead of local time #9025: Session timestamps display in UTC instead of local time - Related locale/timezone issues
• Add option to use 24-hour time format in session view #7208: Add option to use 24-hour time format in session view - Related time format customization
• Updating a session's metadata ought not update the timestamp #9494: Updating a session's metadata ought not update the timestamp - Related timestamp handling concerns
• Zen: Workspace - Missing 31 Jan data, incorrect label in Asia/Singapore timezone in "Usage costs broken down by model" #11606: Timezone-specific date display issues in usage costs (Asia/Singapore timezone)

Feel free to ignore if none of these address your specific case.

[ariane-emory]

This seems undesirable to me. I want OpenCode to format the dates in my country's date format, not in some other international format that I do not use.