Redesign for 1-task-1-process with SQLite status DB

- Simplify to 1-task-1-process model (no file locking needed)
- Add SQLite database (tasks.db) at root for task status management
- Remove .lock files and heartbeat mechanisms
- Focus on implementation details: class design, method specifications
- Add detailed processing flows for task lifecycle, LLM calls, compression
- Specify exact class responsibilities and method behaviors in Japanese
- Include 6-week implementation phases

Co-authored-by: notfolder <20558197+notfolder@users.noreply.github.com>

Author: Copilot