feat(skills): validate manifest installer specs

src/skills/index.ts

@@ -1,4 +1,14 @@
-export type { SkillTier, SkillRequirements, SkillManifest, Skill } from './types.js';
+export type {
+  SkillTier,
+  SkillRequirements,
+  SkillManifest,
+  Skill,
+  SkillInstallerSpec,
+  BrewInstallerSpec,
+  NodeInstallerSpec,
+  GoInstallerSpec,
+  DownloadInstallerSpec,
+} from './types.js';
 export { checkRequirements, loadSkill, discoverSkills, loadAllSkills } from './loader.js';
 export { SkillRegistry } from './registry.js';
 export { SkillInstaller } from './installer.js';