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/adapters/trash/windows/powershell-connection.lua

@@ -41,23 +41,23 @@ function PowershellConnection:_init(init_command)
   -- 65001 is the UTF-8 codepage
   -- powershell needs to be launched with the UTF-8 codepage to use it for both stdin and stdout
   local jid = vim.fn.jobstart({
-    "cmd",
-    "/c",
+    'cmd',
+    '/c',
     '"chcp 65001 && powershell -NoProfile -NoLogo -ExecutionPolicy Bypass -NoExit -Command -"',
   }, {
     ---@param data string[]
     on_stdout = function(_, data)
       for _, fragment in ipairs(data) do
-        if fragment:find("===DONE%((%a+)%)===") then
+        if fragment:find('===DONE%((%a+)%)===') then
           self.is_reading_data = false
-          local output = table.concat(self.stdout, "")
+          local output = table.concat(self.stdout, '')
           local cb = self.commands[1].cb
           table.remove(self.commands, 1)
-          local success = fragment:match("===DONE%((%a+)%)===")
-          if success == "True" then
+          local success = fragment:match('===DONE%((%a+)%)===')
+          if success == 'True' then
             cb(nil, output)
-          elseif success == "False" then
-            cb(success .. ": " .. output, output)
+          elseif success == 'False' then
+            cb(success .. ': ' .. output, output)
           end
           self.stdout = {}
           self:_consume()