refactor(build): simplify dependency detection logic

- Removes unnecessary marker constant from build.zig that existed
  solely to signal build root status
- Uses filesystem check (@src().file access) instead of compile-time
  declaration lookup to detect when ghostty is a dependency
- Same behavior with less indirection: file resolves from build root
  only when ghostty is the main project

build.zig

@@ -318,8 +318,3 @@ pub fn build(b: *std.Build) !void {
         try translations_step.addError("cannot update translations when i18n is disabled", .{});
     }
 }
-
-/// Marker used by Config.zig to detect if ghostty is the build root.
-/// This avoids running logic such as Git tag checking when Ghostty
-/// is used as a dependency.
-pub const _ghostty_build_root = true;

src/build/Config.zig

@@ -219,20 +219,14 @@ pub fn init(b: *std.Build, appVersion: []const u8) !Config {
     else version: {
         const app_version = try std.SemanticVersion.parse(appVersion);
 
-        // Detect if ghostty is being built as a dependency by checking if the
-        // build root has our marker. When used as a dependency, we skip git
-        // detection entirely to avoid reading the downstream project's git state.
-        const is_dependency = !@hasDecl(
-            @import("root"),
-            "_ghostty_build_root",
-        );
-        if (is_dependency) {
-            break :version .{
+        // Is ghostty a dependency? If so, skip git detection.
+        // @src().file won't resolve from b.build_root unless ghostty
+        // is the project being built.
+        b.build_root.handle.access(@src().file, .{}) catch break :version .{
             .major = app_version.major,
             .minor = app_version.minor,
             .patch = app_version.patch,
         };
-        }
 
         // If no explicit version is given, we try to detect it from git.
         const vsn = GitVersion.detect(b) catch |err| switch (err) {