Add tree, inspect, link-child, and unlink-child subcommands to the
agents resource CLI group. Extend ResourceRegistryService with
link_child, unlink_child, get_children, and get_resource_tree methods.
- tree: display resource hierarchy with --depth, --type, and format options
- inspect: show resource details with --tree and --file options
- link-child/unlink-child: manage DAG edges with cycle detection
- 24 Behave scenarios, 4 Robot integration tests, ASV benchmarks
- CLI reference documentation in docs/reference/resource_cli.md
