## Bug: Control UI Crashes When Session Conversation History Is Too Large

### Environment
- **OpenClaw Version**: 2026.3.12
- **Node Version**: v24.14.0
- **OS**: Darwin 25.2.0 (arm64) - macOS
- **Browser**: Chrome / Safari
- **Deployment Mode**: local (localhost)

### Problem Description
The Control UI (Web Chat interface) fails to load properly when a session has many conversation, UI Crashes。 Manifests as page freeze, unresponsive, or crash.

### Steps to Reproduce
1. Start OpenClaw Gateway: `openclaw gateway start`
2. Have a multi-turn conversation via any channel (Feishu/Telegram/Webchat)
3. After 50-200 messages accumulate, refresh the Control UI in browser (usually http://localhost:18789)
4. Observe page loading behavior

### Test Data

**Session File Sizes:**
| Session ID | File Size | Lines | Status |
|------------|-----------|-------|--------|
| 07f595c0-... | 591 KB | 201 | obsolete |
| 1a61747b-... | 247 KB | 149 | main session |
| b07973c7-... | 185 KB | 72 | current active |

**sessions.json Contents:**
- Contains complete metadata for 2 sessions
- Each session stores full `systemPromptReport` (~30KB each)
- `systemPromptReport.tools.schemaChars`: 27,115 characters
- `systemPromptReport.systemPrompt.chars`: 31,666 characters

**sessions Directory Total Size:**
- Before cleanup: ~3.2MB
- After cleanup: ~500KB

### Root Cause Analysis

1. **No Pagination**
   - UI loads and renders all sessions at once
   - Missing `limit` / `offset` pagination support

2. **Redundant Data in sessions.json**
   - Each session stores complete `systemPromptReport`
   - Contains 37 tools' complete schema (27KB)
   - Unnecessary for UI list display

3. **No Virtual Scrolling**
   - When session count > 10, too many DOM nodes
   - No virtual scrolling or windowing implementation

### Expected Fix

**Option A: Reduce Loaded Data**
- Remove `systemPromptReport` field from sessions.json
- Or create lightweight session list API

**Option B: Add Pagination**
- UI adds pagination, load 10-20 sessions at a time
- Gateway supports `?page=1&limit=20` parameter

**Option C: Virtual Scrolling**
- Use react-virtual / vue-virtual-scroller
- Only render visible DOM nodes

### Temporary Workaround

Manually delete obsolete session files:
```bash
rm /Users/hd/.openclaw/agents/main/sessions/*.jsonl
# Keep sessions.json and active session