refactor: revert module namespace from canola back to oil

Problem: the canola rename creates unnecessary friction for users
migrating from stevearc/oil.nvim — every `require('oil')` call and
config reference must change.

Solution: revert all module paths, URL schemes, autocmd groups,
highlight groups, and filetype names back to `oil`. The repo stays
`canola.nvim` for identity; the code is a drop-in replacement.

lua/resession/extensions/oil.lua

@@ -1,7 +1,7 @@
 local M = {}
 
 M.is_win_supported = function(winid, bufnr)
-  return vim.bo[bufnr].filetype == 'canola'
+  return vim.bo[bufnr].filetype == 'oil'
 end
 
 M.save_win = function(winid)
@@ -11,7 +11,7 @@ M.save_win = function(winid)
 end
 
 M.load_win = function(winid, config)
-  require('canola').open(config.bufname)
+  require('oil').open(config.bufname)
 end
 
 return M