Allow expanding ToolCard to show sub-agent processing session

[huypham37]

Context

UX/observability enhancement for AI tool calls with sub-agents.

Problem

When the AI delegates to sub-agents, user has no visibility into what the sub-agent is doing.

Proposal

• ToolCard becomes expandable when it represents a sub-agent call
• Expanding shows the sub-agent's session/conversation
• User can observe sub-agent reasoning and progress
• Provides transparency into delegated work

Acceptance Criteria

• ToolCard has expand/collapse chevron for sub-agent calls
• Expanded view shows sub-agent session content
• Real-time updates as sub-agent processes
• Collapsible to reduce noise when not needed