Add: activeRootRunId to useRun and useJobStream

[FL4TLiN3]

Summary

• Adds activeRootRunId state to useRun() hook, set from startRun events where delegatedBy is absent
• Exposes activeRootRunId from useJobStream() for consumers (Studio)
• Available immediately when SSE events arrive, before run tree API poll returns

Motivation

Studio's conversation view has a race condition: currentRunId depends on useRunTree API poll (~300ms), but SSE startRun provides the runId at ~10ms. activeRootRunId bridges this gap for streaming display (not checkpoint queries, which would 404 since the run isn't in DB yet when SSE fires).

Detection logic

After processRunEventToActivity processes a startRun event, inspect stateRef.current.runStates for the new run. If !runState.delegatedBy, it's a root run → set activeRootRunId. Same pattern as existing isComplete detection.

• Delegated child runs: have delegatedBy → filtered out
• Delegation returns: emit resumeFromStop (not startRun) → never reach this code
• Continuation: new root startRun with status: "completed" → correctly updates

Test plan

• Initial state is null
• Root startRun → sets activeRootRunId
• Delegated startRun → does NOT change activeRootRunId
• Continuation startRun → updates to new runId
• All 26 existing tests pass

🤖 Generated with Claude Code