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/diagnostics.lua

@@ -16,3 +16,8 @@ vim.diagnostic.config({
     },
   },
 })
+
+-- Nvim already binds ]d / [d to jump and <C-W>d to show the diagnostic under
+-- the cursor; these put the same actions on <leader> so which-key lists them.
+vim.keymap.set('n', '<leader>dd', vim.diagnostic.open_float, { desc = 'Show diagnostic' })
+vim.keymap.set('n', '<leader>dq', vim.diagnostic.setqflist, { desc = 'Diagnostics to quickfix' })