chore: consolidate ROADMAP into TASKS.md

- Add future items from ROADMAP to TASKS.md (Voice Enhancements,
  Multi-Tool Expansion, Core Enhancements)
- Delete ROADMAP.json and ROADMAP.md (redundant with TASKS.md)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: grokify