Add `plan explain` and `plan tree` CLI commands that format decision
trees in json/yaml/table/rich formats. Flags control views for
superseded decisions, context snapshots, and reasoning details.
- plan explain <decision_id>: renders a single decision with optional
--show-context, --show-reasoning, --show-alternatives flags
- plan tree <plan_id>: renders full decision tree with optional
--show-superseded and --depth flags
- BFS tree building uses collections.deque (no list.pop(0))
- Behave BDD scenarios (14 scenarios, 54 steps)
- Robot Framework smoke tests with helper script
- ASV benchmarks for explain formatting and tree operations
- Updated docs/reference/plan_cli.md and CHANGELOG.md
ISSUES CLOSED: #174
khyari hamza