build: replace luacheck with selene, add nix devshell and pre-commit (#20)

* build: replace luacheck with selene

Problem: luacheck is unmaintained (last release 2018) and required
suppressing four warning classes to avoid false positives. It also
lacks first-class vim/neovim awareness.

Solution: switch to selene with std='vim' for vim-aware linting.
Replace the luacheck CI job with selene, update the Makefile lint
target, and delete .luacheckrc.

* build: add nix devshell and pre-commit hooks

Problem: oil.nvim had no reproducible dev environment. The .envrc
set up a Python venv for the now-removed docgen pipeline, and there
were no pre-commit hooks for local formatting checks.

Solution: add flake.nix with stylua, selene, and prettier in the
devshell. Replace the stale Python .envrc with 'use flake'. Add
.pre-commit-config.yaml with stylua and prettier hooks matching
other plugins in the repo collection.

* fix: format with stylua

* build(selene): configure lints and add inline suppressions

Problem: selene fails on 5 errors and 3 warnings from upstream code
patterns that are intentional (mixed tables in config API, unused
callback parameters, identical if branches for readability).

Solution: globally allow mixed_table and unused_variable (high volume,
inherent to the codebase design). Add inline selene:allow directives
for the 8 remaining issues: if_same_then_else (4), mismatched_arg_count
(1), empty_if (2), global_usage (1). Remove .envrc from tracking.

* build: switch typecheck action to mrcjkb/lua-typecheck-action

Problem: oil.nvim used stevearc/nvim-typecheck-action, which required
cloning the action repo locally for the Makefile lint target. All
other plugins in the collection use mrcjkb/lua-typecheck-action.

Solution: swap to mrcjkb/lua-typecheck-action@v0 for consistency.
Remove the nvim-typecheck-action git clone from the Makefile and
.gitignore. Drop LuaLS from the local lint target since it requires
a full language server install — CI handles it.

lua/oil/pathutil.lua

@@ -3,26 +3,26 @@ local M = {}
 ---@param path string
 ---@return string
 M.parent = function(path)
-  if path == "/" then
-    return "/"
-  elseif path == "" then
-    return ""
-  elseif vim.endswith(path, "/") then
-    return path:match("^(.*/)[^/]*/$") or ""
+  if path == '/' then
+    return '/'
+  elseif path == '' then
+    return ''
+  elseif vim.endswith(path, '/') then
+    return path:match('^(.*/)[^/]*/$') or ''
   else
-    return path:match("^(.*/)[^/]*$") or ""
+    return path:match('^(.*/)[^/]*$') or ''
   end
 end
 
 ---@param path string
 ---@return nil|string
 M.basename = function(path)
-  if path == "/" or path == "" then
+  if path == '/' or path == '' then
     return
-  elseif vim.endswith(path, "/") then
-    return path:match("^.*/([^/]*)/$")
+  elseif vim.endswith(path, '/') then
+    return path:match('^.*/([^/]*)/$')
   else
-    return path:match("^.*/([^/]*)$")
+    return path:match('^.*/([^/]*)$')
   end
 end