fix(commands): handle :e on diffs:// buffers via BufReadCmd

Problem: running :e on a :Gdiff buffer cleared all content because
diffs:// buffers had no BufReadCmd handler. Neovim tried to read the
buffer name as a file path, found nothing on disk, and emptied the
buffer. This affected all three buffer creation paths (gdiff,
gdiff_file, gdiff_section).

Solution: register a BufReadCmd autocmd for diffs://* that parses the
URL and regenerates diff content from git. Change buffer options from
nofile/wipe to nowrite/delete (matching fugitive's approach) so
buffer-local autocmds and variables survive across unload/reload
cycles. Store old filepath as buffer variable for rename support.

plugin/diffs.lua

@@ -23,6 +23,13 @@ vim.api.nvim_create_autocmd('FileType', {
   end,
 })
 
+vim.api.nvim_create_autocmd('BufReadCmd', {
+  pattern = 'diffs://*',
+  callback = function(args)
+    require('diffs.commands').read_buffer(args.buf)
+  end,
+})
+
 vim.api.nvim_create_autocmd('OptionSet', {
   pattern = 'diff',
   callback = function()