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.

lua/canola/constants.lua

@@ -0,0 +1,13 @@
+local M = {}
+
+---Store entries as a list-like table for maximum space efficiency and retrieval speed.
+---We use the constants below to index into the table.
+---@alias canola.InternalEntry {[1]: integer, [2]: string, [3]: canola.EntryType, [4]: nil|table}
+
+-- Indexes into canola.InternalEntry
+M.FIELD_ID = 1
+M.FIELD_NAME = 2
+M.FIELD_TYPE = 3
+M.FIELD_META = 4
+
+return M