Epic: Isolate checkpoint writes per agent namespace and auto-filter UI by selected agent

[noa-lucent]

Summary
Multiple agent instances currently write to the same MongoDB collection and are only filtered by thread_id. When agents call each other and reuse the same thread_id, their writes collide in the UI stream.

Goal

• Isolate checkpoint writes per agent instance using MongoDBSaver namespaces (checkpoint_ns).
• Keep thread_id for cross-agent context but allow additional filter by namespace in backend.
• In UI, automatically filter stream by the currently selected SimpleAgent (no extra input field).

Scope

• Backend: namespaced checkpointer, filters, socket payload, and SimpleAgent compile-time namespace injection.
• UI: auto-filter by selected agent by passing namespace to the stream hook; no additional UI control.

Acceptance Criteria (Epic)

• Two SimpleAgent nodes running concurrently produce checkpoint_writes with distinct checkpoint_ns values (matching node ids).
• UI shows only the messages for the selected agent in the Builder’s Checkpoint tab; switching selection switches the stream accordingly.
• Behavior remains backward compatible when no namespace is supplied (thread-only filtering or all writes).