Fix which-key not seeing gitsigns maps, add diagnostic maps

The gitsigns keymaps were defined in on_attach, so they were
buffer-local and created asynchronously - gitsigns shells out to git
before attaching. which-key had already built its keymap tree for the
buffer by then, and it only invalidates that cache on BufReadPost,
BufNew and LspAttach, so maps added afterwards stayed invisible.
Buf:get() returns cached mode state unless explicitly passed `update`,
which is why re-entering the buffer didn't help either.

Mapping globally instead means the maps exist before which-key builds
any tree. The gitsigns API no-ops in buffers it hasn't attached to
(checked stage/reset/preview/blame/nav in a non-git directory), so the
only change is that the maps now exist everywhere rather than just in
git-tracked buffers.

Also puts the diagnostic float and quickfix list on <leader>d. Nvim
already binds <C-W>d and ]d/[d for these, so this is a rebinding for
discoverability rather than new capability.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neovim/config/plugins/which-key.lua

@@ -4,8 +4,9 @@ local wk = require('which-key')
 
 wk.setup()
 
--- Names for the gitsigns prefixes, so the popup groups them sensibly
+-- Names for the <leader> prefixes, so the popup groups them sensibly
 wk.add({
+  { '<leader>d', group = 'diagnostics' },
   { '<leader>h', group = 'hunks' },
   { '<leader>t', group = 'toggles' },
 })