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/lsp/helpers.lua

@@ -1,7 +1,7 @@
-local config = require("oil.config")
-local fs = require("oil.fs")
-local util = require("oil.util")
-local workspace = require("oil.lsp.workspace")
+local config = require('oil.config')
+local fs = require('oil.fs')
+local util = require('oil.util')
+local workspace = require('oil.lsp.workspace')
 
 local M = {}
 
@@ -12,7 +12,7 @@ M.will_perform_file_operations = function(actions)
   local creates = {}
   local deletes = {}
   for _, action in ipairs(actions) do
-    if action.type == "move" then
+    if action.type == 'move' then
       local src_scheme, src_path = util.parse_url(action.src_url)
       assert(src_path)
       local src_adapter = assert(config.get_adapter_by_scheme(src_scheme))
@@ -20,32 +20,32 @@ M.will_perform_file_operations = function(actions)
       local dest_adapter = assert(config.get_adapter_by_scheme(dest_scheme))
       src_path = fs.posix_to_os_path(src_path)
       dest_path = fs.posix_to_os_path(assert(dest_path))
-      if src_adapter.name == "files" and dest_adapter.name == "files" then
+      if src_adapter.name == 'files' and dest_adapter.name == 'files' then
         moves[src_path] = dest_path
-      elseif src_adapter.name == "files" then
+      elseif src_adapter.name == 'files' then
         table.insert(deletes, src_path)
-      elseif dest_adapter.name == "files" then
+      elseif dest_adapter.name == 'files' then
         table.insert(creates, src_path)
       end
-    elseif action.type == "create" then
+    elseif action.type == 'create' then
       local scheme, path = util.parse_url(action.url)
       path = fs.posix_to_os_path(assert(path))
       local adapter = assert(config.get_adapter_by_scheme(scheme))
-      if adapter.name == "files" then
+      if adapter.name == 'files' then
         table.insert(creates, path)
       end
-    elseif action.type == "delete" then
+    elseif action.type == 'delete' then
       local scheme, path = util.parse_url(action.url)
       path = fs.posix_to_os_path(assert(path))
       local adapter = assert(config.get_adapter_by_scheme(scheme))
-      if adapter.name == "files" then
+      if adapter.name == 'files' then
         table.insert(deletes, path)
       end
-    elseif action.type == "copy" then
+    elseif action.type == 'copy' then
       local scheme, path = util.parse_url(action.dest_url)
       path = fs.posix_to_os_path(assert(path))
       local adapter = assert(config.get_adapter_by_scheme(scheme))
-      if adapter.name == "files" then
+      if adapter.name == 'files' then
         table.insert(creates, path)
       end
     end
@@ -84,7 +84,7 @@ M.will_perform_file_operations = function(actions)
   accum(workspace.will_rename_files(moves, { timeout_ms = timeout_ms }))
   if final_err then
     vim.notify(
-      string.format("[lsp] file operation error: %s", vim.inspect(final_err)),
+      string.format('[lsp] file operation error: %s', vim.inspect(final_err)),
       vim.log.levels.WARN
     )
   end
@@ -102,7 +102,7 @@ M.will_perform_file_operations = function(actions)
       local bufnr = vim.uri_to_bufnr(uri)
       local was_open = buf_was_modified[bufnr] ~= nil
       local was_modified = buf_was_modified[bufnr]
-      local should_save = autosave == true or (autosave == "unmodified" and not was_modified)
+      local should_save = autosave == true or (autosave == 'unmodified' and not was_modified)
       -- Autosave changed buffers if they were not modified before
       if should_save then
         vim.api.nvim_buf_call(bufnr, function()