feat: add backend plan mode checkpoints

Add backend support for plan-mode turn shaping and exit-plan review
checkpoints. Run-turn commands now accept an interaction mode,
plan-mode prompt guidance tells agents to produce a plan and call
exit_plan_mode, and the sidecar emits a structured
exit-plan-mode-requested event when the SDK raises that session event.

For now the backend intentionally treats exit_plan_mode as a turn
boundary and graceful-degradation review checkpoint. The current
Agent Framework GitHubCopilotAgent wrapper does not expose a clean way
to bridge the live CopilotSession back into same-turn exit-plan
resolution, so that follow-up remains documented for the frontend and a
future backend slice.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Aryx.AgentHost/Services/CopilotTurnExecutionState.cs

@@ -24,6 +24,8 @@ internal sealed class CopilotTurnExecutionState
 
     public List<ChatMessageDto> CompletedMessages { get; private set; } = [];
 
+    public bool HasPendingExitPlanModeRequest { get; private set; }
+
     public async Task EmitThinkingIfNeeded(
         AgentIdentity agent,
         Func<AgentActivityEventDto, Task> onActivity)
@@ -74,6 +76,10 @@ internal sealed class CopilotTurnExecutionState
             case AssistantReasoningDeltaEvent:
                 ActiveAgent = agent;
                 break;
+            case ExitPlanModeRequestedEvent:
+                HasPendingExitPlanModeRequest = true;
+                ActiveAgent = agent;
+                break;
         }
     }