refactor(ai): collapse parser prompts to template; replace makeEmptyItem with structuredClone

- Replace pdf-parser-system.md + docx-parser-system.md with a single
  parser-system.md template; prompts.ts substitutes 6 placeholders per
  source type. Produced strings are byte-identical to the former files.
- Remove makeEmptyItem recursive walker (all SECTION_ITEM_SHAPES leaves are
  already zero-valued) and call structuredClone(shape) instead.

Claude-Session: https://claude.ai/code/session_012Bnvt1MghwHj4qQRxuQUGa

packages/ai/src/resume/extraction-template.ts

@@ -1,23 +1,5 @@
 import { defaultResumeData } from "@reactive-resume/schema/resume/default";
 
-function makeEmptyItem(shape: Record<string, unknown>): Record<string, unknown> {
-	const item: Record<string, unknown> = {};
-
-	for (const [key, value] of Object.entries(shape)) {
-		if (typeof value === "string") item[key] = "";
-		else if (typeof value === "number") item[key] = 0;
-		else if (typeof value === "boolean") item[key] = false;
-		else if (Array.isArray(value)) item[key] = [];
-		else if (value !== null && typeof value === "object") {
-			item[key] = makeEmptyItem(value as Record<string, unknown>);
-		} else {
-			item[key] = "";
-		}
-	}
-
-	return item;
-}
-
 const SECTION_ITEM_SHAPES: Record<string, Record<string, unknown>> = {
 	profiles: {
 		id: "",
@@ -114,7 +96,7 @@ export function buildAiExtractionTemplate() {
 		const sectionKey = key as keyof typeof defaultResumeData.sections;
 		sections[key] = {
 			...defaultResumeData.sections[sectionKey],
-			items: [makeEmptyItem(shape)],
+			items: [structuredClone(shape)],
 		};
 	}