[FEATURE]: Display subagent status in TUI prompt footer

[areyouok]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently, when using subagents in OpenCode, there is no visual indication in the TUI prompt footer about how many subagents are active or exist in total. Users have to manually navigate to the subagent view to check this information.

Proposed enhancement:
Add a subagent status indicator to the prompt footer (bottom-right area next to the "commands" hint) that:

1. Shows nothing when there are no subagents
2. Displays active/total sub agents when subagents exist
3. Shows a ⚡ lightning icon next to the active count when there are active subagents, using theme.warning color to draw attention
4. Uses muted color when no subagents are active

This provides at-a-glance awareness of background subagent activity without leaving the main prompt view.

Implementation approach:

• Compute recursive descendant count (including nested subagents) from sync.data.session using BFS traversal
• Check session_status for "busy" or "retry" states to determine active count
• Conditionally render in the prompt footer using SolidJS <Show>

Screenshots:

With active subagent (⚡ icon + warning color):

Without active subagent (muted color):

[github-actions]

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

• [FEATURE]: Improve subagent runtime visibility in chat UI #22233: Improve subagent runtime visibility in chat UI — requests similar at-a-glance subagent status information in the UI
• [FEATURE]:Show subagent token usage in TUI token counter #22103: Show subagent token usage in TUI token counter — related request for surfacing subagent activity in TUI status areas
• [FEATURE]: Clarify session timing in the UI, especially when subagents are used #22959: Clarify session timing in the UI, especially when subagents are used — overlapping goal of making subagent activity more visible