refactor(coding-agent): unify SkillWarning and ResourceDiagnostic

- Create shared diagnostics.ts with ResourceCollision and ResourceDiagnostic types
- Update loadSkills() to return diagnostics instead of warnings
- Remove SkillWarning type and skillWarningsToDiagnostics() conversion
- Update skills.test.ts to use diagnostics

packages/coding-agent/src/index.ts

@@ -187,7 +187,6 @@ export {
 	loadSkillsFromDir,
 	type Skill,
 	type SkillFrontmatter,
-	type SkillWarning,
 } from "./core/skills.js";
 // Tools
 export {