Add `command="agents plan tree"` parameter to the `format_output()` call for
the `plan tree` CLI command, so that JSON and YAML output conforms to the
spec's Output Rendering Framework. The output is now a proper envelope with
`command`, `status`, `exit_code`, `data`, `timing`, and `messages` fields.
Update BDD step definitions to validate envelope structure and update feature
scenarios for spec-compliant assertions. Remove `@tdd_expected_fail` tag from
the previously-failing JSON tree format test (issue #4254).
ISSUES CLOSED: #11041
controller-ci-rerun
HAL9000