Subagents of a tab's agent should not pin to sidebar as separate tabs

[danshapiro]

Problem

When an agent running in a tab spawns subagents (e.g., Claude Code launching Task tool agents), those subagent sessions appear as separate entries pinned on the left-hand sidebar. They should not — only the top-level agent session that owns the tab should be considered "in a tab" for purposes of sidebar pinning.

Expected Behavior

• A tab running an agent (e.g., Claude Code) pins that agent's session to the sidebar
• Subagents spawned by that agent are not independently pinned to the sidebar
• The sidebar only reflects top-level, user-initiated tab sessions

Current Behavior

Subagent sessions are treated as being "in a tab" and get pinned to the sidebar alongside the parent agent, cluttering the sidebar with entries the user didn't directly create.