fix(plan): wrap plan tree JSON/YAML output in spec-required command envelope

- Wrap `agents plan tree --format json/yaml` output in the spec-required
  command envelope with `command`, `status`, `exit_code`, `data`, `timing`,
  and `messages` fields
- Add summary statistics to output (nodes, depth, child_plans, invariants,
  superseded)
- Implement decision_ids mapping for decision type tracking
- Add child_plans list with associated plan metadata
- Update BDD tests: remove TDD expected_fail tag and fix assertions

ISSUES CLOSED: #9163

CONTRIBUTORS.md

@@ -28,6 +28,7 @@ Below are some of the specific details of various contributions.
 * HAMZA KHYARI has contributed the ACMS execute-phase context assembler project-level hot_max_tokens fix (PR #11036 / issue #11035): added `_resolve_effective_budget()` method that reads each linked project's `settings.hot_max_tokens` and uses the maximum override value as the pipeline budget instead of the hardcoded global 16K default.
 * This project was made possible thanks to considerable donation of time, money, and resources by CleverThis, Inc.
 * HAL 9000 has contributed automated bug fixes, CLI output formatting improvements, and ongoing maintenance as part of the CleverAgents automation system.
+* HAL 9000 has contributed the plan tree JSON/YAML command envelope fix (#9163): wrapped `agents plan tree --format json/yaml` output in the spec-required command envelope structure, added summary statistics, decision_ids mapping, child_plans list, and accurate timing measurement.
 * HAL 9000 has contributed the file edit encoding parameter fix (PR #8258 / issue #7559).
 * HAL 9000 has contributed the architecture-pool-supervisor milestone assignment feature (PR #8188 / issue #7521): added `forgejo_update_pull_request` permission and documented the PR workflow for major spec changes, enabling automatic milestone assignment for specification PRs.
 * HAL 9000 has contributed the git worktree TOCTOU race condition fix (PR #8178 / issue #7507): replaced the unsafe mkdtemp() + rmdir() pattern with a parent-directory approach to eliminate the race window in concurrent git worktree operations.