fix(decisions): add error handling, ULID validation, and CHANGELOG entry to plan tree command

- Remove redundant local import of get_container (already imported at module level)
- Add _validate_plan_ulid() call before querying decision service for actionable error messages
- Wrap service calls in try/except catching ValidationError, PlanError, CleverAgentsError
- Add CHANGELOG.md entry for agents plan tree command (#8525)

ISSUES CLOSED: #8525

CHANGELOG.md

@@ -512,6 +512,14 @@ _ALL_DATA_COLUMNS + ") " "SELECT " + _ALL_DATA_COLUMNS + " FROM v3_plans"`.
   plan/action/project/global configurations are tested with BDD scenarios. 
   Resolution chain is logged at debug level via the
   `PrecedenceResolution` dataclass and `PrecedenceSource` enum.
+- **Plan Tree CLI Command** (#8525): Implemented `agents plan tree <plan-id>` command for
+  visualizing decision trees in the v3 plan lifecycle. The command renders a hierarchical
+  tree structure showing decision hierarchy with per-type ordinal labeling, superseded
+  decision filtering (via `--show-superseded`), depth limiting (via `--depth`), and
+  multiple output formats (rich, plain, table, json, yaml). Each node displays decision
+  ID, type, question, and chosen option. Corrected nodes are visually marked via the
+  `is_superseded` flag. The command handles empty decision trees gracefully and includes
+  ULID validation and proper error handling consistent with other plan commands.
 
 - `agents actor context clear` command to reset actor message history and
   state while preserving the underlying context directory via `ContextManager`