Add Retry for tasks that failed before branch creation

[fedacking]

Problem

When a task fails during the creating_agent step (e.g., the container fails to start), no branch is created. The existing Reopen button cannot be used because it requires a branch_name to be set on the task:

let branch_name = task
    .branch_name
    .as_deref()
    .ok_or_else(|| AppError::BadRequest("Task has no branch — cannot reopen".into()))?;

The only workaround today is to create a new task with the same prompt, losing the original task's context and history.

Proposed Solution

Add a Retry action that re-runs the full task pipeline (container creation → clone → Claude execution) for tasks that failed before a branch was created. This is distinct from Reopen, which only re-enters the follow-up loop on an existing branch.

Considerations:

• Show "Retry" instead of "Reopen" when branch_name is NULL and status is failed
• Reuse the original task ID, prompt, and settings
• Clear the previous error message and logs on retry