fix(ci): switch typecheck action to stevearc/nvim-typecheck-action (#26)

Problem: mrcjkb/lua-typecheck-action runs lua-language-server in a bare
nix sandbox without neovim installed, causing 71 type errors for all
vim.* and uv.* types. LuaLS 3.17.x also introduced stricter type
checking that flags uv.aliases.fs_types mismatches not present in
3.16.4. The .luarc.json workspace.library entries conflicted with the
action's auto-appended luvit-meta, producing duplicate uv type unions.

Solution: switch to stevearc/nvim-typecheck-action@v2 (matching
upstream), pin LuaLS to 3.16.4, convert .luarc.json to nested format
without workspace.library (let the action provide VIMRUNTIME and
luvit-meta), and add .direnv/* to selene.toml exclude for local use.

.luarc.json

@@ -1,8 +1,15 @@
 {
-  "runtime.version": "Lua 5.1",
-  "runtime.path": ["lua/?.lua", "lua/?/init.lua"],
-  "diagnostics.globals": ["vim", "jit", "bit"],
-  "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library"],
-  "workspace.checkThirdParty": false,
-  "completion.callSnippet": "Replace"
+  "runtime": {
+    "version": "LuaJIT",
+    "pathStrict": true
+  },
+  "workspace": {
+    "checkThirdParty": false
+  },
+  "type": {
+    "checkTableShape": true
+  },
+  "completion": {
+    "callSnippet": "Replace"
+  }
 }