Add display name field for tasks #20801

Description

Introduce a new "Display Name" attribute for tasks that serves as a user-friendly, readable name. The display name should be:

• Generated by Claude, in addition to the current task name generation
• As similar as possible to the task name to help users easily identify which workspace belongs to which task
• The primary name shown across all task views (task list, sidebar, and workspace views)

As part of this change, remove the task- prefix from task names (and consequently workspace names) to maintain closer similarity between the task name and display name.

Fallback behavior:

• If Claude generation fails (due to unavailability or error), fallback to generating both name and display name using the task's initial prompt
• If that also fails, use the generated fallback name (similar to workspace name generation)