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({})`.

spec/config_spec.lua

@@ -1,27 +1,15 @@
-local config = require('canola.config')
+local config = require('oil.config')
 
 describe('config', function()
-  after_each(function()
-    vim.g.canola = nil
-  end)
-
-  it('falls back to vim.g.canola when setup() is called with no args', function()
-    vim.g.canola = { delete_to_trash = true, cleanup_delay_ms = 5000 }
-    config.setup()
-    assert.is_true(config.delete_to_trash)
-    assert.equals(5000, config.cleanup_delay_ms)
-  end)
-
-  it('uses defaults when neither opts nor vim.g.canola is set', function()
-    vim.g.canola = nil
+  it('uses defaults when setup() is called with no args', function()
     config.setup()
     assert.is_false(config.delete_to_trash)
     assert.equals(2000, config.cleanup_delay_ms)
   end)
 
-  it('prefers explicit opts over vim.g.canola', function()
-    vim.g.canola = { delete_to_trash = true }
-    config.setup({ delete_to_trash = false })
-    assert.is_false(config.delete_to_trash)
+  it('applies explicit opts', function()
+    config.setup({ delete_to_trash = true, cleanup_delay_ms = 5000 })
+    assert.is_true(config.delete_to_trash)
+    assert.equals(5000, config.cleanup_delay_ms)
   end)
 end)