feat: add confirmation dialog to task kill command

[bborn]

Summary

• Add confirmation dialog before killing a task session (pressing k in detail view)
• On kill confirmation: interrupt task (set status to backlog), log the kill action, and terminate the tmux window
• The kill command now properly stops the Claude process and updates the task status, rather than just closing the tmux window UI

Changes

• Add ViewKillConfirm view state and Kill key binding (k)
• Add showKillConfirm, viewKillConfirm, updateKillConfirm functions for confirmation flow
• Add killTask function that interrupts the task and kills the tmux window
• Add taskKilledMsg message type for handling kill completion
• Clean up detail view panes before executing kill
• Remove direct k key handling from detail.go (now handled at app level with confirmation)

Test plan

• Start a task that creates a Claude session
• Press k in detail view - should show kill confirmation dialog
• Cancel the dialog with Escape - should return to detail view without killing
• Press k again and confirm - should kill session and move task to backlog
• Verify the task log shows "→ [Kill] Session terminated"
• Verify the tmux window for the task is closed

🤖 Generated with Claude Code