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/package.json

@@ -43,6 +43,7 @@
 		"@mariozechner/pi-agent-core": "^0.46.0",
 		"@mariozechner/pi-ai": "^0.46.0",
 		"@mariozechner/pi-tui": "^0.46.0",
+		"@silvia-odwyer/photon-node": "^0.3.4",
 		"chalk": "^5.5.0",
 		"cli-highlight": "^2.1.11",
 		"diff": "^8.0.2",
@@ -51,7 +52,7 @@
 		"marked": "^15.0.12",
 		"minimatch": "^10.1.1",
 		"proper-lockfile": "^4.1.2",
-		"@silvia-odwyer/photon-node": "^0.3.4"
+		"yaml": "^2.8.2"
 	},
 	"devDependencies": {
 		"@types/diff": "^7.0.2",