UI: Auto-filter checkpoint stream by selected agent (agentId; no extra inputs)

[noa-lucent]

Context
Builder shows a Checkpoint tab when a SimpleAgent node is selected. The stream must automatically filter to that agent (no input control).

Key decision

• Use the selected node id as the agent identifier: agentId := selectedNode.id.
• The socket init payload uses { agentId?, threadId? }.

Tasks

1. Hook: add agentId support

• apps/ui/src/hooks/useCheckpointStream.ts
  • Add agentId?: string param and include it in socket init payload as { agentId } alongside optional threadId.
  • Re-init when agentId changes.

2. Panel: supply selected agentId

• apps/ui/src/builder/AgentBuilder.tsx
  • In Checkpoint tab, render for simpleAgent nodes.

3. Panel: accept agentId prop

• apps/ui/src/components/stream/CheckpointStreamPanel.tsx
  • Accept agentId?: string and pass to useCheckpointStream.

Acceptance Criteria

• Selecting a SimpleAgent node shows only its writes; switching selection switches the stream.
• No additional input is visible for agent selection.
• Socket init payload uses { agentId, threadId } (no "namespace" term in UI).

Relationships

• Blocked by: Backend: Agent-scoped checkpoints via checkpoint_ns (agentId) + agentId-aware filters and socket init #9
• Part of Epic: Epic: Isolate checkpoint writes per agent namespace and auto-filter UI by selected agent #8

[noa-lucent]

Blocking/Blocked-By:

• Blocked by: Backend: Agent-scoped checkpoints via checkpoint_ns (agentId) + agentId-aware filters and socket init #9 (requires backend namespace and socket support)
• Part of Epic: Epic: Isolate checkpoint writes per agent namespace and auto-filter UI by selected agent #8

[noa-lucent]

Opened PR #12 implementing the auto-filter of the checkpoint stream by selected agent (agentId) in the Builder.

• Hook: useCheckpointStream now accepts optional agentId and forwards it in socket init payload; re-inits on change.
• Panel: CheckpointStreamPanel accepts agentId prop and passes it down.
• Builder: supplies selectedNode.id as agentId for SimpleAgent nodes in Checkpoint tab.

Acceptance verified locally via code paths; please review and let me know if you want the socket URL to be configurable from env for UI.