Summary

• Fixes [bug]exit_loop with Escalate=true stops parent SequentialAgent pipeline #522: exit_loop with Escalate=true was propagating to parent agents, causing SequentialAgent (which is internally a LoopAgent) to also exit and skip
  remaining sub-agents
• Introduces a dedicated ExitLoop field on EventActions, decoupling loop exit from the Escalate flag
• LoopAgent consumes the ExitLoop flag before yielding events upstream, so parent agents are unaffected
• Escalate retains its original semantics ("escalate to a higher level agent") and passes through loops without triggering exit

Test plan

• Updated loopagent tests to use ExitLoop instead of Escalate
• Added TestSequentialAgentWithNestedLoop verifying Sequential(Loop(..., exit_loop), Summary) continues to Summary
• All existing tests pass (go test ./...)