refactor: rename oil to canola across entire codebase

Problem: the codebase still used the upstream \`oil\` naming everywhere —
URL schemes, the \`:Oil\` command, highlight groups, user events, module
paths, filetypes, buffer/window variables, LuaCATS type annotations,
vimdoc help tags, syntax groups, and internal identifiers.

Solution: mechanical rename of every reference. URL schemes now use
\`canola://\` (plus \`canola-ssh://\`, \`canola-s3://\`, \`canola-sss://\`,
\`canola-trash://\`, \`canola-test://\`). The \`:Canola\` command replaces
\`:Oil\`. All highlight groups, user events, augroups, namespaces,
filetypes, require paths, type annotations, help tags, and identifiers
follow suit. The \`upstream\` remote to \`stevearc/oil.nvim\` has been
removed and the \`vim.g.oil\` deprecation shim dropped.

syntax/canola.vim

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

syntax/canola_preview.vim

@@ -0,0 +1,15 @@
+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

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

syntax/oil_preview.vim

@@ -1,15 +0,0 @@
-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"