Dashboard renderer needs hints for registering subtasks

[melek]

Problem

When creating tasks with subtasks, the session JSON schema supports two formats:

1. Nested subtasks array on the parent task object
2. parent field (integer index into the tasks array) on child tasks

The dashboard renderer (render_dashboard.py) only supports format #2 (the parent field approach). Format #1 (nested subtasks) is silently ignored — the parent renders but children don't appear.

Expected behavior

Either:

• Document the correct format clearly (the parent field approach) so the model uses it consistently, OR
• Support both formats in the renderer, OR
• Have wpl add support a --parent flag for creating subtasks, so the model doesn't need to write raw JSON

Context

The wpl CLI doesn't have a subtask creation command, so the model writes session JSON directly to create parent/child relationships. Without clear guidance on which schema to use, it defaults to the nested format (which is a natural data modeling choice) rather than the index-based parent field the renderer expects.

Suggestion

Add a note to the state schema docs about the parent field format, and consider adding wpl add "subtask title" --parent t2 to the CLI.