Tasks API: Implement GET /tasks/{id}/logs #901

Implement logs retrieval endpoint.

Base path: /api/v2/organizations/{org}/members/{user}

Behavior

• Query: follow, tail.
• Connect to agentapi to fetch history/output as logs.
• Handle empty/missing logs gracefully.
• Non-goal: no streaming (SSE/WebSocket) or webhooks, polling only for MVP.

References: #838