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/test/fixtures/skills/invalid-yaml/SKILL.md

@@ -0,0 +1,8 @@
+---
+name: invalid-yaml
+description: [unclosed bracket
+---
+
+# Invalid YAML Skill
+
+This skill has invalid YAML in the frontmatter.

packages/coding-agent/test/fixtures/skills/multiline-description/SKILL.md

@@ -0,0 +1,11 @@
+---
+name: multiline-description
+description: |
+  This is a multiline description.
+  It spans multiple lines.
+  And should be normalized.
+---
+
+# Multiline Description Skill
+
+This skill tests that multiline YAML descriptions are normalized to single lines.

packages/coding-agent/test/frontmatter.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it } from "vitest";
+import { parseFrontmatter, stripFrontmatter } from "../src/utils/frontmatter.js";
+
+describe("parseFrontmatter", () => {
+	it("parses keys, strips quotes, and returns body", () => {
+		const input = "---\nname: \"skill-name\"\ndescription: 'A desc'\nfoo-bar: value\n---\n\nBody text";
+		const { frontmatter, body } = parseFrontmatter<Record<string, string>>(input);
+		expect(frontmatter.name).toBe("skill-name");
+		expect(frontmatter.description).toBe("A desc");
+		expect(frontmatter["foo-bar"]).toBe("value");
+		expect(body).toBe("Body text");
+	});
+
+	it("normalizes newlines and handles CRLF", () => {
+		const input = "---\r\nname: test\r\n---\r\nLine one\r\nLine two";
+		const { body } = parseFrontmatter<Record<string, string>>(input);
+		expect(body).toBe("Line one\nLine two");
+	});
+
+	it("throws on invalid YAML frontmatter", () => {
+		const input = "---\nfoo: [bar\n---\nBody";
+		expect(() => parseFrontmatter<Record<string, string>>(input)).toThrow(/at line 1, column 10/);
+	});
+
+	it("parses | multiline yaml syntax", () => {
+		const input = "---\ndescription: |\n  Line one\n  Line two\n---\n\nBody";
+		const { frontmatter, body } = parseFrontmatter<Record<string, string>>(input);
+		expect(frontmatter.description).toBe("Line one\nLine two\n");
+		expect(body).toBe("Body");
+	});
+
+	it("returns original content when frontmatter is missing or unterminated", () => {
+		const noFrontmatter = "Just text\nsecond line";
+		const missingEnd = "---\nname: test\nBody without terminator";
+		const resultNoFrontmatter = parseFrontmatter<Record<string, string>>(noFrontmatter);
+		const resultMissingEnd = parseFrontmatter<Record<string, string>>(missingEnd);
+		expect(resultNoFrontmatter.body).toBe("Just text\nsecond line");
+		expect(resultMissingEnd.body).toBe(
+			"---\nname: test\nBody without terminator".replace(/\r\n/g, "\n").replace(/\r/g, "\n"),
+		);
+	});
+
+	it("returns empty object for empty or comment-only frontmatter", () => {
+		const input = "---\n# just a comment\n---\nBody";
+		const { frontmatter } = parseFrontmatter(input);
+		expect(frontmatter).toEqual({});
+	});
+});
+
+describe("stripFrontmatter", () => {
+	it("removes frontmatter and trims body", () => {
+		const input = "---\nkey: value\n---\n\nBody\n";
+		expect(stripFrontmatter(input)).toBe("Body");
+	});
+
+	it("returns body when no frontmatter present", () => {
+		const input = "\n  No frontmatter body  \n";
+		expect(stripFrontmatter(input)).toBe("\n  No frontmatter body  \n");
+	});
+});

packages/coding-agent/test/skills.test.ts

@@ -95,6 +95,28 @@ describe("skills", () => {
 			expect(warnings.some((w) => w.message.includes("description is required"))).toBe(true);
 		});
 
+		it("should warn and skip skill when YAML frontmatter is invalid", () => {
+			const { skills, warnings } = loadSkillsFromDir({
+				dir: join(fixturesDir, "invalid-yaml"),
+				source: "test",
+			});
+
+			expect(skills).toHaveLength(0);
+			expect(warnings.some((w) => w.message.includes("at line"))).toBe(true);
+		});
+
+		it("should preserve multiline descriptions from YAML", () => {
+			const { skills, warnings } = loadSkillsFromDir({
+				dir: join(fixturesDir, "multiline-description"),
+				source: "test",
+			});
+
+			expect(skills).toHaveLength(1);
+			expect(skills[0].description).toContain("\n");
+			expect(skills[0].description).toContain("This is a multiline description.");
+			expect(warnings).toHaveLength(0);
+		});
+
 		it("should warn when name contains consecutive hyphens", () => {
 			const { skills, warnings } = loadSkillsFromDir({
 				dir: join(fixturesDir, "consecutive-hyphens"),