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/git.lua

@@ -0,0 +1,118 @@
+-- integration with git operations
+local fs = require('oil.fs')
+
+local M = {}
+
+---@param path string
+---@return string|nil
+M.get_root = function(path)
+  local git_dir = vim.fs.find('.git', { upward = true, path = path })[1]
+  if git_dir then
+    return vim.fs.dirname(git_dir)
+  else
+    return nil
+  end
+end
+
+---@param path string
+---@param cb fun(err: nil|string)
+M.add = function(path, cb)
+  local root = M.get_root(path)
+  if not root then
+    return cb()
+  end
+
+  local stderr = ''
+  local jid = vim.fn.jobstart({ 'git', 'add', path }, {
+    cwd = root,
+    stderr_buffered = true,
+    on_stderr = function(_, data)
+      stderr = table.concat(data, '\n')
+    end,
+    on_exit = function(_, code)
+      if code ~= 0 then
+        cb('Error in git add: ' .. stderr)
+      else
+        cb()
+      end
+    end,
+  })
+  if jid <= 0 then
+    cb()
+  end
+end
+
+---@param path string
+---@param cb fun(err: nil|string)
+M.rm = function(path, cb)
+  local root = M.get_root(path)
+  if not root then
+    return cb()
+  end
+
+  local stderr = ''
+  local jid = vim.fn.jobstart({ 'git', 'rm', '-r', path }, {
+    cwd = root,
+    stderr_buffered = true,
+    on_stderr = function(_, data)
+      stderr = table.concat(data, '\n')
+    end,
+    on_exit = function(_, code)
+      if code ~= 0 then
+        stderr = vim.trim(stderr)
+        if stderr:match("^fatal: pathspec '.*' did not match any files$") then
+          cb()
+        else
+          cb('Error in git rm: ' .. stderr)
+        end
+      else
+        cb()
+      end
+    end,
+  })
+  if jid <= 0 then
+    cb()
+  end
+end
+
+---@param entry_type oil.EntryType
+---@param src_path string
+---@param dest_path string
+---@param cb fun(err: nil|string)
+M.mv = function(entry_type, src_path, dest_path, cb)
+  local src_git = M.get_root(src_path)
+  if not src_git or src_git ~= M.get_root(dest_path) then
+    fs.recursive_move(entry_type, src_path, dest_path, cb)
+    return
+  end
+
+  local stderr = ''
+  local jid = vim.fn.jobstart({ 'git', 'mv', src_path, dest_path }, {
+    cwd = src_git,
+    stderr_buffered = true,
+    on_stderr = function(_, data)
+      stderr = table.concat(data, '\n')
+    end,
+    on_exit = function(_, code)
+      if code ~= 0 then
+        stderr = vim.trim(stderr)
+        if
+          stderr:match('^fatal: not under version control')
+          or stderr:match('^fatal: source directory is empty')
+        then
+          fs.recursive_move(entry_type, src_path, dest_path, cb)
+        else
+          cb('Error in git mv: ' .. stderr)
+        end
+      else
+        cb()
+      end
+    end,
+  })
+  if jid <= 0 then
+    -- Failed to run git, fall back to normal filesystem operations
+    fs.recursive_move(entry_type, src_path, dest_path, cb)
+  end
+end
+
+return M