Fix undo restoring todo list (#4081)

[riatzukiza]

Summary

• capture todowrite output during undo and restore it on cleanup so undo affects the plan (undo does not undo todo list, resulting in agents continuing on the same path as the undo #4081)
• add a regression test that simulates todowrite messages to ensure the todo list rolls back

Testing

• bun turbo typecheck
• bun test packages/opencode/test/session/revert-todo.test.ts

[riatzukiza]

The todo list is updated, but the tui doesn't show the update until after a new message is sent.

[riatzukiza]

should be good now