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.