[FEATURE]: Manual todo management

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently the todo list can only be modified by the agent. If the agent forgets to update or clear todos, users have no way to fix stale entries.

I'd like to add manual todo management. One possible interface would be a /todo command, though I'm happy to follow whatever interaction pattern fits OpenCode best:

• "Toggle todo"
• "Edit todo"
• "Delete todo"
• "Clear all todos"

This wouldn't change the todowrite behavior, only provide a manual fallback for user-facing state.

If this direction sounds good, I'm happy to open a PR.