feat: Add /subagents command to view subagent tree

[adam2am]

Problem

There's no way to see the full subagent hierarchy in one place.

Current Navigation Options

1. <leader>+←/→ — Cycles through subagents one-by-one

   • Gets clunky fast with many subagents
   • No overview, just sequential stepping

2. <leader>+↑ — Goes back to root session

   • Only option to "escape" the subagent tree
   • Can't jump to siblings or specific children

3. Session list — Shows main sessions only (subagents are filtered out)

   • By design, to avoid clutter
   • But leaves no dedicated place to manage subagents

When This Hurts Most

• Async multi-agent workflows (e.g., oh-my-opencode spawning parallel agents)
• Cleaning up leftovers — need to find and delete stale subagents
• Checking specific subagent details — cycling through 10+ agents to find one

Proposed Solution

Add /subagents command showing:

• Full tree structure from root
• Status indicators (idle/busy/retry)
• Direct navigation (click to switch)
• Delete (ctrl+d) and rename (ctrl+r) matching session list UX

This keeps the main session list clean while giving subagents a dedicated home.

Implementation

PR: #6289

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Add interactive session switcher for navigating between subagent sessions #6183: Add interactive session switcher for navigating between subagent sessions

Both issues propose showing the full subagent hierarchy in one place with status indicators, direct navigation, and the ability to delete/rename sessions. #6183 suggests using a keybind-triggered dialog, while this issue proposes a /subagents command. These could potentially be consolidated into a single feature that supports both access methods.