macos: add build script, update AGENTS.md, skip UI tests

This is an update to address common agentic issues I run into,
but the `build.nu` script may be generally helpful to people using
the Nix env since `xcodebuild` is broken by default in Nix due to the
compiler/linker overrides Nix shell does.

src/build/GhosttyXcodebuild.zig

@@ -104,6 +104,8 @@ pub fn init(
             "test",
             "-scheme",
             "Ghostty",
+            "-skip-testing",
+            "GhosttyUITests",
         });
         if (xc_arch) |arch| step.addArgs(&.{ "-arch", arch });