Centralize frontmatter parsing + parse frontmatter with yaml library (#728)

* Add frontmatter utility and tidy coding agent prompts

* Add frontmatter parsing utilities and tests

* Parse frontmatter with YAML parser

* Simplify frontmatter parsing utilities

* strip body in 1 place

* Improve frontmatter parsing error handling

* Normalize multiline skill and select-list descriptions

packages/coding-agent/src/utils/frontmatter.ts

@@ -0,0 +1,39 @@
+import { parse } from "yaml";
+
+type ParsedFrontmatter<T extends Record<string, unknown>> = {
+	frontmatter: T;
+	body: string;
+};
+
+const normalizeNewlines = (value: string): string => value.replace(/\r\n/g, "\n").replace(/\r/g, "\n");
+
+const extractFrontmatter = (content: string): { yamlString: string | null; body: string } => {
+	const normalized = normalizeNewlines(content);
+
+	if (!normalized.startsWith("---")) {
+		return { yamlString: null, body: normalized };
+	}
+
+	const endIndex = normalized.indexOf("\n---", 3);
+	if (endIndex === -1) {
+		return { yamlString: null, body: normalized };
+	}
+
+	return {
+		yamlString: normalized.slice(4, endIndex),
+		body: normalized.slice(endIndex + 4).trim(),
+	};
+};
+
+export const parseFrontmatter = <T extends Record<string, unknown> = Record<string, unknown>>(
+	content: string,
+): ParsedFrontmatter<T> => {
+	const { yamlString, body } = extractFrontmatter(content);
+	if (!yamlString) {
+		return { frontmatter: {} as T, body };
+	}
+	const parsed = parse(yamlString);
+	return { frontmatter: (parsed ?? {}) as T, body };
+};
+
+export const stripFrontmatter = (content: string): string => parseFrontmatter(content).body;