feat: add arrow key shortcuts to navigate tasks in detail view

[bborn]

Summary

• When viewing task details, pressing ↑/↓ arrow keys (or j/k) navigates to the previous/next task in the same kanban column
• Updates the help bar to show "↑/↓ prev/next task" instead of "↑/↓ scroll"
• Properly cleans up tmux panes before switching to the new task

Test plan

• Open a task from a kanban column with multiple tasks
• Press ↑ or k to navigate to the previous task in the column
• Press ↓ or j to navigate to the next task in the column
• Verify the detail view updates to show the new task
• Verify navigation wraps around (at the bottom, pressing ↓ goes to the top)
• Verify tmux panes are properly cleaned up and recreated when switching tasks

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