Feat | App-of-apps traversal support

[dag-andersen]

Summary

• Adds --traverse-app-of-apps flag that recursively discovers and renders child Applications/ApplicationSets found in rendered manifests (the app-of-apps pattern)
• Uses content similarity (instead of positional pairing) to correctly match duplicate-identity apps in diff output, fixing false delete+add for same-name apps with different content (e.g. prod vs test variants)
• Includes spec hash in the visited key so same-name apps with different spec.source.path are rendered separately during traversal

Other Changes

• Integration tests - Two new branch-17 test cases (root-only and full-folder app-of-apps)
• Documentation - docs/app-of-apps.md with usage guide and filter behavior notes