refactor: replace x-opencode-* headers with x-kilo-* across codebase and upstream scripts

Update upstream merge scripts to use a generic x-opencode- → x-kilo- regex
that catches all current and future headers automatically. Apply the same
replacement to all source files so the codebase is ready for the next
upstream merge.

script/upstream/codemods/transform-strings.ts

@@ -40,6 +40,9 @@ const STRING_REPLACEMENTS: StringReplacement[] = [
   // Binary name references (be careful with these)
   { pattern: /\bopencode upgrade\b/g, replacement: "kilo upgrade" },
 
+  // HTTP header prefix
+  { pattern: /x-opencode-/g, replacement: "x-kilo-" },
+
   // Environment variables (exclude OPENCODE_API_KEY - upstream Zen SaaS key)
   { pattern: /\bOPENCODE_(?!API_KEY\b)([A-Z_]+)\b/g, replacement: "KILO_$1" },
   { pattern: /\bVITE_OPENCODE_/g, replacement: "VITE_KILO_" },

script/upstream/transforms/transform-take-theirs.ts

@@ -122,9 +122,9 @@ const BRANDING_REPLACEMENTS: BrandingReplacement[] = [
     description: "Window global",
   },
   {
-    pattern: /x-opencode-client/g,
-    replacement: "x-kilo-client",
-    description: "HTTP header",
+    pattern: /x-opencode-/g,
+    replacement: "x-kilo-",
+    description: "HTTP header prefix",
   },
   {
     pattern: /_EXTENSION_OPENCODE_/g,