Evil-mode keymapping being overridden by org-mode

[Brandon Amos <bamos@cs.cmu.edu>]

Hi, I am mapping the "t" character in evil normal/motion modes to
`evil-next-line` with the following. I am doing this mapping
for reasonable navigation with the Dvorak layout.

  (define-key evil-normal-state-map "t" 'evil-next-line)
  (define-key evil-motion-state-map "t" 'evil-next-line)

This works well, except in one case when org mode over-rides
my mapping with `org-todo`:

  1. Open Emacs and an org file
  2. In normal mode, `C-h k t` shows that `t` is bound to evil-next-line.
  3. Open the agenda with `C-a a`
  4. Close the agenda buffer with `C-x 0`
  5. In normal mode, `C-h k t` shows that `t` is bound to `org-todo`

How can I prevent opening the agenda buffer from
over-writing this key mapping like this?

I've tried searching for ways to unset this org-mode keyboard mapping
and ways to define keyboard mappings that can't be over-ridden,
but I haven't been able to find anything that works.

If it is helpful, I am using this version of spacemacs:
https://github.com/syl20bnr/spacemacs/tree/c2774bc0dea3dbc63621709d54685fbbe9212d55
My (near-vanilla) spacemacs init file is here.
I set the evil keymappings in dotspacemacs/user-config:
https://github.com/bamos/dotfiles/blob/master/.spacemacs.d/init.el

-Brandon.