feat(coding-agent): add packages array with filtering support

- Add PackageSource type for npm/git sources with optional filtering
- Migrate npm:/git: sources from extensions to packages array
- Add getPackages(), setPackages(), setProjectPackages() methods
- Update package-manager to resolve from packages array
- Support selective loading: extensions, skills, prompts, themes per package
- Update pi list to show packages
- Add migration tests for settings

closes #645

packages/coding-agent/src/index.ts

@@ -168,6 +168,7 @@ export {
 export {
 	type CompactionSettings,
 	type ImageSettings,
+	type PackageSource,
 	type RetrySettings,
 	SettingsManager,
 } from "./core/settings-manager.js";