Remove vendored headshots and add proportion CI tests

- Remove Knowles_Headshot_2019_good.jpg and Knowles_Headshot_2019_prisma.jpg
  from inst/img/ — personal headshot should not be distributed with the package
- Update plot_jpeg() roxygen example to use a generic placeholder instead of
  the removed headshot path; regenerate man/plot_jpeg.Rd accordingly
- Add comprehensive testthat coverage for clopper_pearson(), z_univariate(),
  waldInterval(), and agresti_coull_interval() in tests/testthat/test_propint.R

The propint module previously had zero tests. New tests cover: return types,
formula correctness (cross-validated against binom.test()), edge cases
(0/n and n/n), interval validity, confidence level behavior, and sign/direction
of z-scores.

man/plot_jpeg.Rd

@@ -22,7 +22,8 @@ Plot a jpeg image as a raster
 }
 \examples{
 \dontrun{
-img <- system.file("img","Knowles_Headshot_2019_good.jpg",package="civilytics")
+# Supply a path to your own JPEG file
+img <- "my_photo.jpg"
 plot_jpeg(img)
 }
 }