show runtime logs for WebSocket agents

[zatevakhin]

Summary

Make agent Runtime/System stdout and stderr logs available for WebSocket agents.

Session debug events already work over stdio and WebSocket. The missing part is runtime logging, which currently depends on the local sidecar.

Scope

• Expose bounded runtime logs through an ACP capability.
• Reuse the existing searchable log UI with filtering, follow, and copy.
• Handle disconnects, truncation, and unsupported agents clearly.
• Do not restore /ui/ws.