current_datetime injected as UTC — should use local timezone

[jmathes]

The <current_datetime> tag injected into every user message always uses UTC (Z suffix), regardless of system timezone. The model consistently misinterprets local time as a result. It can be partially fixed with custom instructions, but this is more expensive and less reliable.

Proposed fix: Use timezone-aware offset (-08:00) instead of Z. One-line change from toISOString() to local-aware formatting.

Affects: file/folder date naming, log timestamp interpretation, ETA estimation, contextual time awareness ("Saturday evening" vs "Sunday morning").