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/core/diagnostics.ts

@@ -0,0 +1,15 @@
+export interface ResourceCollision {
+	resourceType: "extension" | "skill" | "prompt" | "theme";
+	name: string; // skill name, command/tool/flag name, prompt name, theme name
+	winnerPath: string;
+	loserPath: string;
+	winnerSource?: string; // e.g., "npm:foo", "git:...", "local"
+	loserSource?: string;
+}
+
+export interface ResourceDiagnostic {
+	type: "warning" | "error" | "collision";
+	message: string;
+	path?: string;
+	collision?: ResourceCollision;
+}