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/keymap_util.lua

@@ -1,7 +1,7 @@
-local actions = require("oil.actions")
-local config = require("oil.config")
-local layout = require("oil.layout")
-local util = require("oil.util")
+local actions = require('oil.actions')
+local config = require('oil.config')
+local layout = require('oil.layout')
+local util = require('oil.util')
 local M = {}
 
 ---@param rhs string|table|fun()
@@ -9,14 +9,14 @@ local M = {}
 ---@return table opts
 ---@return string|nil mode
 local function resolve(rhs)
-  if type(rhs) == "string" and vim.startswith(rhs, "actions.") then
-    local action_name = vim.split(rhs, ".", { plain = true })[2]
+  if type(rhs) == 'string' and vim.startswith(rhs, 'actions.') then
+    local action_name = vim.split(rhs, '.', { plain = true })[2]
     local action = actions[action_name]
     if not action then
-      vim.notify("[oil.nvim] Unknown action name: " .. action_name, vim.log.levels.ERROR)
+      vim.notify('[oil.nvim] Unknown action name: ' .. action_name, vim.log.levels.ERROR)
     end
     return resolve(action)
-  elseif type(rhs) == "table" then
+  elseif type(rhs) == 'table' then
     local opts = vim.deepcopy(rhs)
     -- We support passing in a `callback` key, or using the 1 index as the rhs of the keymap
     local callback, parent_opts = resolve(opts.callback or opts[1])
@@ -25,17 +25,17 @@ local function resolve(rhs)
     if parent_opts.desc and not opts.desc then
       if opts.opts then
         opts.desc =
-          string.format("%s %s", parent_opts.desc, vim.inspect(opts.opts):gsub("%s+", " "))
+          string.format('%s %s', parent_opts.desc, vim.inspect(opts.opts):gsub('%s+', ' '))
       else
         opts.desc = parent_opts.desc
       end
     end
 
     local mode = opts.mode
-    if type(rhs.callback) == "string" then
+    if type(rhs.callback) == 'string' then
       local action_opts, action_mode
       callback, action_opts, action_mode = resolve(rhs.callback)
-      opts = vim.tbl_extend("keep", opts, action_opts)
+      opts = vim.tbl_extend('keep', opts, action_opts)
       mode = mode or action_mode
     end
 
@@ -46,7 +46,7 @@ local function resolve(rhs)
     opts.deprecated = nil
     opts.parameters = nil
 
-    if opts.opts and type(callback) == "function" then
+    if opts.opts and type(callback) == 'function' then
       local callback_args = opts.opts
       opts.opts = nil
       local orig_callback = callback
@@ -68,7 +68,7 @@ M.set_keymaps = function(keymaps, bufnr)
   for k, v in pairs(keymaps) do
     local rhs, opts, mode = resolve(v)
     if rhs then
-      vim.keymap.set(mode or "", k, rhs, vim.tbl_extend("keep", { buffer = bufnr }, opts))
+      vim.keymap.set(mode or '', k, rhs, vim.tbl_extend('keep', { buffer = bufnr }, opts))
     end
   end
 end
@@ -95,9 +95,9 @@ M.show_help = function(keymaps)
     local all_lhs = lhs_to_all_lhs[k]
     if all_lhs then
       local _, opts = resolve(rhs)
-      local keystr = table.concat(all_lhs, "/")
+      local keystr = table.concat(all_lhs, '/')
       max_lhs = math.max(max_lhs, vim.api.nvim_strwidth(keystr))
-      table.insert(keymap_entries, { str = keystr, all_lhs = all_lhs, desc = opts.desc or "" })
+      table.insert(keymap_entries, { str = keystr, all_lhs = all_lhs, desc = opts.desc or '' })
     end
   end
   table.sort(keymap_entries, function(a, b)
@@ -108,20 +108,20 @@ M.show_help = function(keymaps)
   local highlights = {}
   local max_line = 1
   for _, entry in ipairs(keymap_entries) do
-    local line = string.format(" %s   %s", util.pad_align(entry.str, max_lhs, "left"), entry.desc)
+    local line = string.format(' %s   %s', util.pad_align(entry.str, max_lhs, 'left'), entry.desc)
     max_line = math.max(max_line, vim.api.nvim_strwidth(line))
     table.insert(lines, line)
     local start = 1
     for _, key in ipairs(entry.all_lhs) do
       local keywidth = vim.api.nvim_strwidth(key)
-      table.insert(highlights, { "Special", #lines, start, start + keywidth })
+      table.insert(highlights, { 'Special', #lines, start, start + keywidth })
       start = start + keywidth + 1
     end
   end
 
   local bufnr = vim.api.nvim_create_buf(false, true)
   vim.api.nvim_buf_set_lines(bufnr, 0, -1, true, lines)
-  local ns = vim.api.nvim_create_namespace("Oil")
+  local ns = vim.api.nvim_create_namespace('Oil')
   for _, hl in ipairs(highlights) do
     local hl_group, lnum, start_col, end_col = unpack(hl)
     vim.api.nvim_buf_set_extmark(bufnr, ns, lnum - 1, start_col, {
@@ -129,21 +129,21 @@ M.show_help = function(keymaps)
       hl_group = hl_group,
     })
   end
-  vim.keymap.set("n", "q", "<cmd>close<CR>", { buffer = bufnr })
-  vim.keymap.set("n", "<c-c>", "<cmd>close<CR>", { buffer = bufnr })
+  vim.keymap.set('n', 'q', '<cmd>close<CR>', { buffer = bufnr })
+  vim.keymap.set('n', '<c-c>', '<cmd>close<CR>', { buffer = bufnr })
   vim.bo[bufnr].modifiable = false
-  vim.bo[bufnr].bufhidden = "wipe"
+  vim.bo[bufnr].bufhidden = 'wipe'
 
   local editor_width = vim.o.columns
   local editor_height = layout.get_editor_height()
   local winid = vim.api.nvim_open_win(bufnr, true, {
-    relative = "editor",
+    relative = 'editor',
     row = math.max(0, (editor_height - #lines) / 2),
     col = math.max(0, (editor_width - max_line - 1) / 2),
     width = math.min(editor_width, max_line + 1),
     height = math.min(editor_height, #lines),
     zindex = 150,
-    style = "minimal",
+    style = 'minimal',
     border = config.keymaps_help.border,
   })
   local function close()
@@ -151,13 +151,13 @@ M.show_help = function(keymaps)
       vim.api.nvim_win_close(winid, true)
     end
   end
-  vim.api.nvim_create_autocmd("BufLeave", {
+  vim.api.nvim_create_autocmd('BufLeave', {
     callback = close,
     once = true,
     nested = true,
     buffer = bufnr,
   })
-  vim.api.nvim_create_autocmd("WinLeave", {
+  vim.api.nvim_create_autocmd('WinLeave', {
     callback = close,
     once = true,
     nested = true,