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/ITurnWorkflowRunner.cs

@@ -10,6 +10,7 @@ public interface ITurnWorkflowRunner
         Func<AgentActivityEventDto, Task> onActivity,
         Func<ApprovalRequestedEventDto, Task> onApproval,
         Func<UserInputRequestedEventDto, Task> onUserInput,
+        Func<ExitPlanModeRequestedEventDto, Task> onExitPlanMode,
         CancellationToken cancellationToken);
 
     Task ResolveApprovalAsync(