refactor: revert canola namespace to oil and remove vim.g config (#120)

* 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.

* refactor: remove `vim.g.oil` declarative config

Problem: the `vim.g.oil` configuration path was added prematurely.
It adds a second config entrypoint before the plugin has stabilized
enough to justify it.

Solution: remove `vim.g.oil` support from `plugin/oil.lua`,
`config.setup()`, docs, and tests. Users configure via
`require("oil").setup({})`.

lua/resession/extensions/oil.lua

@@ -0,0 +1,17 @@
+local M = {}
+
+M.is_win_supported = function(winid, bufnr)
+  return vim.bo[bufnr].filetype == 'oil'
+end
+
+M.save_win = function(winid)
+  local bufnr = vim.api.nvim_win_get_buf(winid)
+  local bufname = vim.api.nvim_buf_get_name(bufnr)
+  return { bufname = bufname }
+end
+
+M.load_win = function(winid, config)
+  require('oil').open(config.bufname)
+end
+
+return M