[Feature] Custom command to cycle between child sessions #5578

Summary

Add a custom slash command (e.g., /cs or /child) to cycle through child sessions, providing an alternative to the keybind approach which can conflict with terminal emulators.

Problem

The current keybinds for cycling child sessions (<leader>+right / <leader>+left) don't work reliably across all terminal emulators. Some terminals capture arrow key combinations before OpenCode receives them.

Proposed Solution

Add a slash command that provides child session navigation:

/cs           # Show list of child sessions with picker
/cs next      # Cycle to next child session
/cs prev      # Cycle to previous child session  
/cs 1         # Jump to child session 1
/cs list      # List all child sessions

Alternatively, extend the existing /session command:

/session children     # List child sessions
/session child next   # Cycle next
/session child 2      # Jump to specific child

Use Case

When using the Task tool, multiple child sessions are spawned. Users need a reliable way to:

1. See what child sessions exist
2. Switch between them to monitor progress
3. Review results from completed child tasks

Additional Context

This would complement the existing keybind approach and provide a fallback for users whose terminal setups conflict with the default keybinds.