fix(date): ensure Linux date utility returns UTC timestamps

Add -u flag to GNU date command in get_iso_timestamp() to match
the macOS implementation and function documentation. Both platforms
now consistently return UTC timestamps in YYYY-MM-DDTHH:MM:SS+00:00
format.

Before: date -Iseconds (returned local time on Linux)
After:  date -u -Iseconds (returns UTC on Linux)

lib/date_utils.sh

@@ -14,8 +14,8 @@ get_iso_timestamp() {
         # Use manual formatting and add colon to timezone offset
         date -u +"%Y-%m-%dT%H:%M:%S%z" | sed 's/\(..\)$/:\1/'
     else
-        # Linux (GNU date)
-        date -Iseconds
+        # Linux (GNU date) - use -u flag for UTC
+        date -u -Iseconds
     fi
 }