Marks the jj status line with a colored 'jj' powerline pill so it stands
out from command output above it. Track FiraCode Nerd Font in the Brewfile
since the powerline separators need a Nerd Font.
Bold magenta on the unique prefix, gray on the rest, matching jj's own
log output. Gray parens instead of yellow. Empty marker moved to the
diff-stat slot for a uniform layout.