feat(skills): add dry-run installer planning surface

src/skills/index.ts

@@ -10,6 +10,8 @@ export type {
   DownloadInstallerSpec,
 } from './types.js';
 export { checkRequirements, loadSkill, discoverSkills, loadAllSkills } from './loader.js';
+export { buildInstallerPlan } from './planner.js';
+export type { InstallerPlan, InstallerPlanStep, InstallerPlanSkip, InstallerPlanningOptions } from './planner.js';
 export { SkillRegistry } from './registry.js';
 export { SkillInstaller } from './installer.js';
 export { SkillsWatcher } from './watcher.js';