feat(difftool): replace old "nvim -d" automatically #38057

Problem:
"nvim -d" doesn't leverage nvim.difftool.

Solution:
If nvim.difftool was enabled via :packadd, automatically
handle "nvim -d" on startup.

    nvim -c "packadd nvim.difftool" -d dir1/ dir2/

test/functional/plugin/difftool_spec.lua

@@ -115,4 +115,35 @@ describe('nvim.difftool', function()
     local autocmds = fn.nvim_get_autocmds({ group = 'nvim.difftool.events' })
     assert(#autocmds > 0, 'autocmds should still exist after closing quickfix window')
   end)
+
+  it('opens difftool automatically when started with nvim -d', function()
+    -- Start Neovim with -d flag for directory diff
+    clear({
+      args = {
+        '--cmd',
+        'packadd nvim.difftool',
+        '-d',
+        testdir_left,
+        testdir_right,
+      },
+    })
+
+    -- Wait for difftool to open
+    n.poke_eventloop()
+
+    -- Verify we have 3 windows (left, right, and quickfix)
+    eq(3, #fn.getwininfo())
+
+    -- Verify quickfix list has the expected entries
+    local qflist = fn.getqflist()
+    local entries = {}
+    for _, item in ipairs(qflist) do
+      table.insert(entries, { text = item.text, rel = item.user_data and item.user_data.rel })
+    end
+    eq({
+      { text = 'M', rel = 'file1.txt' },
+      { text = 'D', rel = 'file2.txt' },
+      { text = 'A', rel = 'file3.txt' },
+    }, entries)
+  end)
 end)