chore: remove redundant gitignore entries and simplify INSTALL.md

- Remove ghostty/zig-out/ and ghostty/zig-cache/ from .gitignore
  (submodule contents are automatically ignored by git)
- Simplify INSTALL.md to just run build script
  (script handles submodule initialization automatically)

Author: sreya

.gitignore

@@ -6,7 +6,3 @@ dist/
 
 # WASM build output (built locally and in CI)
 ghostty-vt.wasm
-
-# Ghostty submodule build artifacts
-ghostty/zig-out/
-ghostty/zig-cache/

INSTALL.md

@@ -98,11 +98,10 @@ The library requires the `ghostty-vt.wasm` file at runtime. When installing from
 After cloning:
 
 ```bash
-git submodule update --init --recursive
 ./scripts/build-wasm.sh
 ```
 
-The WASM file is generated locally and gitignored.
+The script will automatically initialize the submodule if needed. The WASM file is generated locally and gitignored.
 
 ### Vite (Recommended)