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/oil/ringbuf.lua

@@ -0,0 +1,37 @@
+---@class oil.Ringbuf
+---@field private size integer
+---@field private tail integer
+---@field private buf string[]
+local Ringbuf = {}
+function Ringbuf.new(size)
+  local self = setmetatable({
+    size = size,
+    buf = {},
+    tail = 0,
+  }, { __index = Ringbuf })
+  return self
+end
+
+---@param val string
+function Ringbuf:push(val)
+  self.tail = self.tail + 1
+  if self.tail > self.size then
+    self.tail = 1
+  end
+  self.buf[self.tail] = val
+end
+
+---@return string
+function Ringbuf:as_str()
+  local postfix = ''
+  for i = 1, self.tail, 1 do
+    postfix = postfix .. self.buf[i]
+  end
+  local prefix = ''
+  for i = self.tail + 1, #self.buf, 1 do
+    prefix = prefix .. self.buf[i]
+  end
+  return prefix .. postfix
+end
+
+return Ringbuf