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({})`.

syntax/canola.vim

@@ -1,7 +0,0 @@
-if exists("b:current_syntax")
-  finish
-endif
-
-syn match canolaId /^\/\d* / conceal
-
-let b:current_syntax = "canola"

syntax/canola_preview.vim

@@ -1,15 +0,0 @@
-if exists("b:current_syntax")
-  finish
-endif
-
-syn match canolaCreate /^CREATE\( BUCKET\)\? /
-syn match canolaMove   /^  MOVE /
-syn match canolaDelete /^DELETE\( BUCKET\)\? /
-syn match canolaCopy   /^  COPY /
-syn match canolaChange /^CHANGE /
-" Trash operations
-syn match canolaRestore /^RESTORE /
-syn match canolaPurge /^ PURGE /
-syn match canolaTrash /^ TRASH /
-
-let b:current_syntax = "canola_preview"

syntax/oil.vim

@@ -0,0 +1,7 @@
+if exists("b:current_syntax")
+  finish
+endif
+
+syn match oilId /^\/\d* / conceal
+
+let b:current_syntax = "oil"

syntax/oil_preview.vim

@@ -0,0 +1,15 @@
+if exists("b:current_syntax")
+  finish
+endif
+
+syn match oilCreate /^CREATE\( BUCKET\)\? /
+syn match oilMove   /^  MOVE /
+syn match oilDelete /^DELETE\( BUCKET\)\? /
+syn match oilCopy   /^  COPY /
+syn match oilChange /^CHANGE /
+" Trash operations
+syn match oilRestore /^RESTORE /
+syn match oilPurge /^ PURGE /
+syn match oilTrash /^ TRASH /
+
+let b:current_syntax = "oil_preview"