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/loading.lua

@@ -1,4 +1,4 @@
-local util = require("oil.util")
+local util = require('oil.util')
 local M = {}
 
 local timers = {}
@@ -12,14 +12,14 @@ M.is_loading = function(bufnr)
 end
 
 local spinners = {
-  dots = { "⠋", "⠙", "⠹", "⠸", "⠼", "⠴", "⠦", "⠧", "⠇", "⠏" },
+  dots = { '⠋', '⠙', '⠹', '⠸', '⠼', '⠴', '⠦', '⠧', '⠇', '⠏' },
 }
 
 ---@param name_or_frames string|string[]
 ---@return fun(): string
 M.get_iter = function(name_or_frames)
   local frames
-  if type(name_or_frames) == "string" then
+  if type(name_or_frames) == 'string' then
     frames = spinners[name_or_frames]
     if not frames then
       error(string.format("Unrecognized spinner: '%s'", name_or_frames))
@@ -35,26 +35,26 @@ M.get_iter = function(name_or_frames)
 end
 
 M.get_bar_iter = function(opts)
-  opts = vim.tbl_deep_extend("keep", opts or {}, {
+  opts = vim.tbl_deep_extend('keep', opts or {}, {
     bar_size = 3,
     width = 20,
   })
   local i = 0
   return function()
-    local chars = { "[" }
+    local chars = { '[' }
     for _ = 1, opts.width - 2 do
-      table.insert(chars, " ")
+      table.insert(chars, ' ')
     end
-    table.insert(chars, "]")
+    table.insert(chars, ']')
 
     for j = i - opts.bar_size, i do
       if j > 1 and j < opts.width then
-        chars[j] = "="
+        chars[j] = '='
       end
     end
 
     i = (i + 1) % (opts.width + opts.bar_size)
-    return table.concat(chars, "")
+    return table.concat(chars, '')
   end
 end
 
@@ -75,7 +75,7 @@ M.set_loading = function(bufnr, is_loading)
             return
           end
           local lines =
-            { util.pad_align("Loading", math.floor(width / 2) - 3, "right"), bar_iter() }
+            { util.pad_align('Loading', math.floor(width / 2) - 3, 'right'), bar_iter() }
           util.render_text(bufnr, lines)
         end)
       )