feat: add interactive session switcher for subagent navigation

[rmk40]

Summary

Adds an interactive session switcher dialog for navigating between parent and subagent sessions, replacing the need to cycle through sessions one-by-one.

• Hierarchical display of parent and descendant sessions (up to 3 levels)
• Permission indicator (◉) for sessions awaiting user input
• Busy spinner for active sessions
• New keybind: <leader>s (session_child_list)
• Appears in suggested commands when subagents exist

Closes #6183

Related Issues

This addresses several related feature requests and pain points:

• Partially addresses #5826 (Agent Session Tab Bar Navigation) - provides visual session overview via dialog
• Partially addresses #6032 (Nerdtree like session manager) - hierarchical display in picker format
• Helps with #4727 (Subagent Switching - Buggy) - direct selection bypasses cycling lag issues
• Relates to #5578 (Custom command to cycle) - accessible via command palette as alternative to keybinds