[Enhancement] No parent/child relationships visible for subagents in sidebar

[YoungVoney]

When a main session spawns subagents, there's no visual indication of the parent-child relationship in the sidebar.

Current Behavior

All sessions appear in a flat list:
OpenClaw (expanded) ├─ telegram:g-agent-main-main ├─ System: [date] Exec... (actually a subagent) ├─ Cron: ClawRouter Wallet Monitor ├─ Cron: Daily Memory Curation └─ Cron: Anti-Todo Hourly Logger

No indication that "System: [date]..." was spawned BY the telegram session.

Expected Behavior

Option A: Tree view with indentation
OpenClaw (expanded) ├─ telegram:g-agent-main-main │ └─ Subagent: test-subagent-demo (spawned from main) ├─ Cron: ClawRouter Wallet Monitor ├─ Cron: Daily Memory Curation └─ Cron: Anti-Todo Hourly Logger

Option B: Collapsible parent with children
OpenClaw (expanded) ├─► telegram:g-agent-main-main (1 subagent) │ └─ Subagent: test-subagent-demo ├─ Cron: ClawRouter Wallet Monitor ├─ Cron: Daily Memory Curation └─ Cron: Anti-Todo Hourly Logger

Option C: Parent context in subagent name
OpenClaw (expanded) ├─ telegram:g-agent-main-main ├─ Subagent: test-subagent-demo (from telegram:g-agent-main-main) ├─ Cron: ClawRouter Wallet Monitor ├─ Cron: Daily Memory Curation └─ Cron: Anti-Todo Hourly Logger