Feature Request: Plan status indicators (badge/symbol) on session list items

### Describe the feature or problem you'd like to solve

When managing multiple sessions, there's no visual way to tell at a glance what stage each session's plan is in. You have to open each session to check progress, which breaks flow when juggling several workstreams.

### Proposed solution

Add a small status indicator (symbol with tooltip, or badge) on each session item in the sidebar that reflects the current plan state:

| Status | Meaning | Suggested Icon |
|--------|---------|----------------|
| **Planning** | Agent is actively generating/refining the plan | 🔄 or spinner |
| **PlanReady** | Plan has been generated and is awaiting user approval or kickoff | 📋 or checkmark-outline |
| **PlanInProgress** | Plan has been approved and work is underway | ⚙️ or progress indicator |
| **PlanComplete** | All plan steps are done | ✅ or filled checkmark |

**Requirements:**
- The indicator should be compact enough to not clutter the session list
- A tooltip on hover should show the full status label (e.g., "Plan In Progress")
- Status should update in real-time as the session progresses
- Sessions without a plan (e.g., chat sessions, sessions in interactive/autopilot mode) can omit the indicator or show a neutral state

### Example prompts or workflows

1. User has 5 project sessions open
2. Glancing at the sidebar, they can immediately see:
- Session A: ✅ (done, ready to review/merge)
- Session B: ⚙️ (agent is executing the plan)
- Session C: 📋 (plan ready, waiting for approval)
- Session D: 🔄 (still planning)
3. User clicks Session C to approve the plan, then moves on — no wasted context-switching

### Additional context

Related to #981 (Status Sidebar). This is a lighter-weight version focused specifically on plan lifecycle visibility in the existing session list UI.

まず、既存のセッションリスト/サイドバーと、セッションで使用されているプランのライフサイクル状態を特定します。計画、承認、実行、完了の更新がどのように伝播されるかを追跡し、そのうえで、コンパクトなインジケーター、ホバーラベル、リアルタイム更新、プランのないセッションが明記された要件を満たしていることを確認します。

索引モデルが issue の本文から書いたものです。