test: add comprehensive tests for new UX features and fix ISO date timezone bug

Add 33 new test functions covering annotations, undo system, history
formatting, relative date display, and weekday parsing pipeline. Fix ISO
date parsing to use ParseInLocation instead of Parse to respect the
parser's timezone context.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/engine/dateparse.go

@@ -54,7 +54,7 @@ func (p *DateParser) parseDateOnly(s string) (time.Time, error) {
 	}
 
 	// Try ISO format first
-	if t, err := time.Parse("2006-01-02", s); err == nil {
+	if t, err := time.ParseInLocation("2006-01-02", s, p.base.Location()); err == nil {
 		return t, nil
 	}