Add diffview.nvim for reviewing diffs and file history

Installed via vim.pack, matching how the other plugins here are
managed. Adds a new <leader>g "diff" group: gd opens a Diffview
against the index, gc closes it, gh shows history for the current
file, gH for the branch.

Verified headlessly: config/plugins loads clean, :DiffviewOpen is
registered, and opening/closing a Diffview against this repo works
with no errors.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013t7aQbc4EyYtab1crvSFTd

neovim/config/plugins/which-key.lua

@@ -8,6 +8,7 @@ wk.setup()
 wk.add({
   { '<leader>d', group = 'diagnostics' },
   { '<leader>f', group = 'find' },
+  { '<leader>g', group = 'diff' },
   { '<leader>h', group = 'hunks' },
   { '<leader>t', group = 'toggles' },
 })