feat: allow GUI task creation params

[michael-chou359]

Summary

• Add an optional + Add task creation params control to the OOB chat prompt that is only shown before a task exists.
• Reveal a JSON editor for task creation params when users opt in, with a remove action that clears stale params.
• Use the provided JSON object as task/create.params when present, while preserving the existing prompt-derived params fallback when empty or not enabled.
• Reset task params UI state after successful task creation, independent of whether the first prompt is text or JSON.
• Add JSON object parsing helpers and unit coverage for empty, invalid, and non-object inputs.

Validation

• npm run lint
• npm test -- --run lib/json-utils.test.ts
• npm run typecheck