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/tests/Aryx.AgentHost.Tests/AgentInstructionComposerTests.cs

@@ -124,6 +124,33 @@ public sealed class AgentInstructionComposerTests
         Assert.DoesNotContain("Do not inspect, modify, create, or delete files", instructions, StringComparison.OrdinalIgnoreCase);
     }
 
+    [Fact]
+    public void Compose_AddsPlanModeGuidanceWhenRequested()
+    {
+        PatternDefinitionDto pattern = new()
+        {
+            Id = "pattern-single",
+            Name = "Single",
+            Mode = "single",
+            Availability = "available",
+        };
+        PatternAgentDefinitionDto agent = CreateAgent(
+            id: "agent-primary",
+            name: "Primary Agent",
+            instructions: "You are a helpful assistant.");
+
+        string instructions = AgentInstructionComposer.Compose(
+            pattern,
+            agent,
+            agentIndex: 0,
+            interactionMode: "plan");
+
+        Assert.Contains("operating in plan mode", instructions, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("produce a concrete implementation plan", instructions, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("exit_plan_mode", instructions, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("Do not continue into implementation", instructions, StringComparison.OrdinalIgnoreCase);
+    }
+
     private static PatternAgentDefinitionDto CreateAgent(string id, string name, string instructions)
     {
         return new PatternAgentDefinitionDto