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/SidecarProtocolHost.cs

@@ -21,7 +21,6 @@ public sealed class SidecarProtocolHost
         AskUserToolName,
         "report_intent",
         "task_complete",
-        "exit_plan_mode",
     };
 
     private static readonly string[] AuthenticationErrorIndicators =
@@ -189,6 +188,7 @@ public sealed class SidecarProtocolHost
                     activity => WriteAsync(context.Output, activity, turnCancellation.Token),
                     approval => WriteAsync(context.Output, approval, turnCancellation.Token),
                     userInput => WriteAsync(context.Output, userInput, turnCancellation.Token),
+                    exitPlanMode => WriteAsync(context.Output, exitPlanMode, turnCancellation.Token),
                     turnCancellation.Token)
                 .ConfigureAwait(false);