Feature Request: Multi-session visibility — no way to monitor parallel agent sessions simultaneously

## Pain Point

When orchestrating multiple AI agent sessions working on different features in parallel, there is **no way to view all sessions at once** in the app.

### Current experience

- The app shows **one session at a time** in the sidebar
- To check on 5+ parallel sessions, I have to click through each one individually
- Opening multiple app instances is not supported (shared state, resource heavy)
- The alternative (tmux with CLI agents) gives visual tiling but loses the app's orchestration, free frontier models, and session management

### What I want

A **mission control / dashboard view** — a single screen showing:
- All active sessions in a tiled or grid layout
- Live status per session (working, idle, blocked, done)
- Recent actions/diffs per session at a glance
- Ability to steer/message any session directly from the overview
- Quick drill-down to full session view

### Visual reference

The ideal UX is similar to tmux with 6-12 panes, each showing a different agent's progress in real-time — but with the orchestration capabilities, plan approval, and model access that this app provides.

### My workflow

I'm a solo developer building a web app (React/TypeScript). I spawn 5-6 parallel sessions working on independent features:
- Session 1: ML-based stem separation
- Session 2: Waveform visualization
- Session 3: Audio-video auto-sync
- Session 4: Playback engine fixes + dead code cleanup
- Session 5: UI copy changes

These all run in autopilot on separate branches. My only option to monitor them is asking the chat for a status update or clicking through each session one by one. I can't see what's happening across all of them simultaneously.

### Proposed solutions (any of these would help)

1. **Dashboard view** in the app — grid/list of all active sessions with live status, progress indicators, and recent activity
2. **Split/tiled view** — show 2-4 sessions side-by-side in the main area
3. **Status widget** — persistent widget above the composer showing all session statuses without navigating away
4. **External API/streaming** — expose session events so users can build their own dashboards or pipe to terminal multiplexers

### Why this matters

Multi-session parallel development is one of the app's strongest capabilities — but the single-session UI becomes the bottleneck once you're actually using it at scale. The agents are fast; the human's ability to observe and steer them is the constraint.

No repository files, tests, or entry points are named. Start by examining the current single-session sidebar and session view, then compare the dashboard, split view, status widget, and external API options against the stated multi-session workflow. Done should provide simultaneous visibility, live status, recent activity, and direct steering or drill-down for active sessions.

索引モデルが issue の本文から書いたものです。