fix(gsc): correct the claim that no off-engine compiler exists

gsc-tool compiles GSC outside the engine and reproduces the engine's own
diagnostics, so the skill's "no compiler, linter, or test harness worth the
name" framing was wrong and sent the reader to load a map instead of running a
50ms check. §1 is now the gate: `--mode comp --game <title> --system pc --dry`,
with the warning that `--mode parse` is syntax-only and accepts code the
compiler rejects, and that `--game` takes the engine codename (h2 for MW2CR)
rather than the marketing name.

A clean compile proves the script loads, not that it works, so the
"behaviour is unverified until the map runs" half is kept and sharpened.

Also adds two §8 entries found the hard way: a local declared inside a block is
not visible after it, and there are no file-scope constants.

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

README.md

@@ -25,7 +25,7 @@ A collection of public, reusable agent skills, cheatsheets, and operational work
 | **C# / .NET** | [`csharp-dotnet.md`](skills-public/code-languages/csharp-dotnet.md) | `TreatWarningsAsErrors`, nullable enabled, no `.Result`, trimming/single-file publish caveats, SDK shims |
 | **C / C++** | [`c-cpp.md`](skills-public/code-languages/c-cpp.md) | full warning set + `-Werror`, sanitizers, RAII/Rule of Zero, target-based CMake, FFI ownership |
 | **Lua** | [`lua.md`](skills-public/code-languages/lua.md) | pin the dialect first, `luac -p` + luacheck as the only gate, locals over globals, `pcall` at the boundary, `#t`/pattern traps, embedded & HavokScript hosts |
-| **GSC** | [`gsc.md`](skills-public/code-languages/gsc.md) | no compiler or test harness, `isdefined` on everything external, `endon` before the first wait, per-frame budget, precache at load, level-state side effects |
+| **GSC** | [`gsc.md`](skills-public/code-languages/gsc.md) | compile off-engine with `gsc-tool --mode comp`, `isdefined` on everything external, `endon` before the first wait, per-frame budget, precache at load, level-state side effects |
 
 Each language file extends `coding-dev.md` rather than repeating it. A project's own
 `AGENTS.md` / `CLAUDE.md` / `.rules` always overrides both.