docs(code-languages): add Lua and GSC skills

Lua covers dialect pinning (5.1/LuaJIT/5.2/5.3+/HavokScript), luac -p and
luacheck as the only real gate for an embedded host, locals over globals,
pcall at the boundary, the #t / pairs / pattern traps, and the sandboxed-host
rules including Call of Duty LUI specifics.

GSC covers the absence of any compiler, linter, or test harness and what to
do instead, scope prefixing on level/self, isdefined on every external read,
endon before the first wait, the per-frame instruction budget, load-phase
precaching, and side effects on shared level state.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

coding-dev.md

@@ -38,6 +38,8 @@ Per-language specifics live in [`code-languages/`](skills-public/code-languages/
 | Go | [`code-languages/go.md`](skills-public/code-languages/go.md) |
 | C# / .NET | [`code-languages/csharp-dotnet.md`](skills-public/code-languages/csharp-dotnet.md) |
 | C / C++ | [`code-languages/c-cpp.md`](skills-public/code-languages/c-cpp.md) |
+| Lua | [`code-languages/lua.md`](skills-public/code-languages/lua.md) |
+| GSC (Call of Duty script) | [`code-languages/gsc.md`](skills-public/code-languages/gsc.md) |
 
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