feat: add /todo slash command

[athal7]

Summary

• Adds /todo built-in slash command to manage the session todo list without interrupting current work
• Closes #4134

Changes

• Add packages/opencode/src/command/template/todo.txt - New prompt template
• Modify packages/opencode/src/command/index.ts - Register the command

Usage

# Add items
/todo check error handling in auth module
/todo 1. fix tests 2. update docs

# Complete items
/todo done auth-error

# Cancel items  
/todo cancel old-task

# List items
/todo list
/todo

The key feature is that the agent will process the todo command and immediately resume previous work without context switching, addressing the core request in #4134.