fix(ci): resolve lua-language-server warnings (#32)

Problem: reload_spec.lua called io.open() without nil checks, causing
need-check-nil warnings. Adding ${3rd}/busted and ${3rd}/luassert to
workspace.library caused lua-language-server 3.7.4 to run diagnostics
on its own bundled meta files, surfacing pre-existing cast-local-type
bugs in luassert's annotations that are not ours to fix.

Solution: use assert(io.open(...)) in reload_spec.lua to satisfy the
nil check. Remove busted/luassert library paths from .luarc.json since
they only benefit spec/ which is not type-checked in CI. Narrow the
lua-language-server check in scripts/ci.sh to lua/ to match CI.

.luarc.json

@@ -2,12 +2,7 @@
   "runtime.version": "LuaJIT",
   "runtime.path": ["lua/?.lua", "lua/?/init.lua"],
   "diagnostics.globals": ["vim", "jit"],
-  "workspace.library": [
-    "$VIMRUNTIME/lua",
-    "${3rd}/luv/library",
-    "${3rd}/busted/library",
-    "${3rd}/luassert/library"
-  ],
+  "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library"],
   "workspace.checkThirdParty": false,
   "workspace.ignoreDir": [".direnv"],
   "completion.callSnippet": "Replace"