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/path_spec.lua

@@ -1,13 +1,13 @@
-local pathutil = require("oil.pathutil")
-describe("pathutil", function()
-  it("calculates parent path", function()
+local pathutil = require('oil.pathutil')
+describe('pathutil', function()
+  it('calculates parent path', function()
     local cases = {
-      { "/foo/bar", "/foo/" },
-      { "/foo/bar/", "/foo/" },
-      { "/", "/" },
-      { "", "" },
-      { "foo/bar/", "foo/" },
-      { "foo", "" },
+      { '/foo/bar', '/foo/' },
+      { '/foo/bar/', '/foo/' },
+      { '/', '/' },
+      { '', '' },
+      { 'foo/bar/', 'foo/' },
+      { 'foo', '' },
     }
     for _, case in ipairs(cases) do
       local input, expected = unpack(case)
@@ -16,12 +16,12 @@ describe("pathutil", function()
     end
   end)
 
-  it("calculates basename", function()
+  it('calculates basename', function()
     local cases = {
-      { "/foo/bar", "bar" },
-      { "/foo/bar/", "bar" },
-      { "/", nil },
-      { "", nil },
+      { '/foo/bar', 'bar' },
+      { '/foo/bar/', 'bar' },
+      { '/', nil },
+      { '', nil },
     }
     for _, case in ipairs(cases) do
       local input, expected = unpack(case)