rfc: get screenshots / recordings after session end

## Summary

Add ability to list and retrieve screenshots/recordings after a shell session ends, useful when:
- LLM forgets filename
- LLM calls `shell_stop` before downloading
- User asks "what screenshots did you take?"

## Proposal

### Option 1: `shell_list_files` tool

```typescript
shell_list_files({
session_id?: string, // Optional - if omitted, list all sessions
type?: "screenshots" | "recordings" | "all" // Default: "all"
})
```

Returns:
```json
{
"files": [
{
"filename": "k9s-services.gif",
"type": "recording",
"download_url": "http://localhost:7498/files/...",
"created_at": "2025-12-17T14:30:00Z",
"size_bytes": 5500000
}
]
}
```

### Option 2: HTTP endpoint

Simple `GET /files` endpoint returning JSON listing of all available files.