[Bug] task_update returns "success" but status doesn't change for branch-created tasks

Bug Description

When calling task_update to change task status from "backlog" to "done", the tool returns "success" but the task status remains unchanged in the dashboard.

Steps to Reproduce

1. Create tasks using branch/spawn process
2. Tasks get created with status "backlog" and worker_id bound after completion
3. Call task_update(task_number=N, status="done")
4. Response returns "success"
5. Re-check task_list - status still shows "backlog"

Expected Behavior

task_update should change the task status from "backlog" to "done" when called with valid parameters.

Actual Behavior

task_update returns "success" but no actual change occurs. The task remains in "backlog" status.

Additional Context

• Tasks created by agents (agent:financial, agent:sport) update correctly
• Tasks created by "branch" created_by field appear to be locked
• completed_at field remains null for affected tasks
• worker_id is set correctly but status doesn't auto-transition

Environment

Spacebot version: 0.3.3

Start by tracing the task_update implementation and the status handling for tasks created through the branch/spawn process, then reproduce the backlog-to-done update from the issue. Done means task_update changes the dashboard and task_list status to done and completed_at is no longer null for affected tasks.