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/index.ts

@@ -276,3 +276,4 @@ export {
 	Theme,
 	type ThemeColor,
 } from "./modes/interactive/theme/theme.js";
+export { parseFrontmatter, stripFrontmatter } from "./utils/frontmatter.js";