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/diffview.lua

@@ -0,0 +1,12 @@
+vim.pack.add({ 'https://github.com/sindrets/diffview.nvim' })
+
+require('diffview').setup()
+
+local function map(lhs, rhs, desc)
+  vim.keymap.set('n', lhs, rhs, { desc = desc })
+end
+
+map('<leader>gd', '<Cmd>DiffviewOpen<CR>', 'Diff against index')
+map('<leader>gc', '<Cmd>DiffviewClose<CR>', 'Close diff view')
+map('<leader>gh', '<Cmd>DiffviewFileHistory %<CR>', 'File history')
+map('<leader>gH', '<Cmd>DiffviewFileHistory<CR>', 'Branch history')

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')

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' },
 })