Stream subagent transcript in real-time within parent transcript view

[amahpour]

Context

Follow-up from #19 (nest subagent sessions inside parent session tile).

Currently, the transcript view shows "Spawned Agent" blocks that render the subagent's full conversation after it completes. The agent tool result arrives as a single chunk — there's no streaming of intermediate progress in the parent's transcript.

Proposed Enhancement

While a subagent is still running (no tool_result yet), show a live-updating view of its conversation within the transcript:

1. Detect [Tool: Agent] entries that don't yet have a corresponding tool_result (agent still in flight)
2. Show a "live" Spawned Agent block with a spinning/pulsing indicator
3. Poll the subagent's transcript endpoint (/api/sessions/agent-{id}/transcript) and stream entries into the block in real-time
4. Once the tool_result arrives, collapse back to the standard summary view

Concerns

• Could be visually noisy with multiple concurrent subagents streaming
• The tile already provides live visibility into active subagents
• The transcript is primarily a historical record — mixing real-time content could make it jumpy

Decision

Deferred as a "nice to have." The current UX split (tile = live dashboard, transcript = history with expandable agent conversations) is clean and sufficient. Revisit if users want deeper real-time visibility without switching between tile and transcript views.