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/init.lua

@@ -1,3 +1,4 @@
+require('config.plugins.diffview')
 require('config.plugins.fzf-lua')
 require('config.plugins.gitsigns')
 require('config.plugins.nvim-tree')