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.

tests/select_spec.lua

@@ -1,22 +1,22 @@
-require("plenary.async").tests.add_to_env()
-local oil = require("oil")
-local test_util = require("tests.test_util")
+require('plenary.async').tests.add_to_env()
+local oil = require('oil')
+local test_util = require('tests.test_util')
 
-a.describe("oil select", function()
+a.describe('oil select', function()
   after_each(function()
     test_util.reset_editor()
   end)
 
-  a.it("opens file under cursor", function()
+  a.it('opens file under cursor', function()
     test_util.oil_open()
     -- Go to the bottom, so the cursor is not on a directory
-    vim.cmd.normal({ args = { "G" } })
+    vim.cmd.normal({ args = { 'G' } })
     a.wrap(oil.select, 2)()
     assert.equals(1, #vim.api.nvim_tabpage_list_wins(0))
-    assert.not_equals("oil", vim.bo.filetype)
+    assert.not_equals('oil', vim.bo.filetype)
   end)
 
-  a.it("opens file in new tab", function()
+  a.it('opens file in new tab', function()
     test_util.oil_open()
     local tabpage = vim.api.nvim_get_current_tabpage()
     a.wrap(oil.select, 2)({ tab = true })
@@ -25,7 +25,7 @@ a.describe("oil select", function()
     assert.not_equals(tabpage, vim.api.nvim_get_current_tabpage())
   end)
 
-  a.it("opens file in new split", function()
+  a.it('opens file in new split', function()
     test_util.oil_open()
     local winid = vim.api.nvim_get_current_win()
     a.wrap(oil.select, 2)({ vertical = true })
@@ -34,9 +34,9 @@ a.describe("oil select", function()
     assert.not_equals(winid, vim.api.nvim_get_current_win())
   end)
 
-  a.it("opens multiple files in new tabs", function()
+  a.it('opens multiple files in new tabs', function()
     test_util.oil_open()
-    vim.api.nvim_feedkeys("Vj", "x", true)
+    vim.api.nvim_feedkeys('Vj', 'x', true)
     local tabpage = vim.api.nvim_get_current_tabpage()
     a.wrap(oil.select, 2)({ tab = true })
     assert.equals(3, #vim.api.nvim_list_tabpages())
@@ -44,9 +44,9 @@ a.describe("oil select", function()
     assert.not_equals(tabpage, vim.api.nvim_get_current_tabpage())
   end)
 
-  a.it("opens multiple files in new splits", function()
+  a.it('opens multiple files in new splits', function()
     test_util.oil_open()
-    vim.api.nvim_feedkeys("Vj", "x", true)
+    vim.api.nvim_feedkeys('Vj', 'x', true)
     local winid = vim.api.nvim_get_current_win()
     a.wrap(oil.select, 2)({ vertical = true })
     assert.equals(1, #vim.api.nvim_list_tabpages())
@@ -54,23 +54,23 @@ a.describe("oil select", function()
     assert.not_equals(winid, vim.api.nvim_get_current_win())
   end)
 
-  a.describe("close after open", function()
-    a.it("same window", function()
-      vim.cmd.edit({ args = { "foo" } })
+  a.describe('close after open', function()
+    a.it('same window', function()
+      vim.cmd.edit({ args = { 'foo' } })
       local bufnr = vim.api.nvim_get_current_buf()
       test_util.oil_open()
       -- Go to the bottom, so the cursor is not on a directory
-      vim.cmd.normal({ args = { "G" } })
+      vim.cmd.normal({ args = { 'G' } })
       a.wrap(oil.select, 2)({ close = true })
       assert.equals(1, #vim.api.nvim_tabpage_list_wins(0))
       -- This one we actually don't expect the buffer to be the same as the initial buffer, because
       -- we opened a file
       assert.not_equals(bufnr, vim.api.nvim_get_current_buf())
-      assert.not_equals("oil", vim.bo.filetype)
+      assert.not_equals('oil', vim.bo.filetype)
     end)
 
-    a.it("split", function()
-      vim.cmd.edit({ args = { "foo" } })
+    a.it('split', function()
+      vim.cmd.edit({ args = { 'foo' } })
       local bufnr = vim.api.nvim_get_current_buf()
       local winid = vim.api.nvim_get_current_win()
       test_util.oil_open()
@@ -79,8 +79,8 @@ a.describe("oil select", function()
       assert.equals(bufnr, vim.api.nvim_win_get_buf(winid))
     end)
 
-    a.it("tab", function()
-      vim.cmd.edit({ args = { "foo" } })
+    a.it('tab', function()
+      vim.cmd.edit({ args = { 'foo' } })
       local bufnr = vim.api.nvim_get_current_buf()
       local tabpage = vim.api.nvim_get_current_tabpage()
       test_util.oil_open()