feat: support project copilot customization

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

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

@@ -151,6 +151,39 @@ public sealed class AgentInstructionComposerTests
         Assert.Contains("Do not continue into implementation", instructions, StringComparison.OrdinalIgnoreCase);
     }
 
+    [Fact]
+    public void Compose_InsertsProjectInstructionsBetweenBaseAndRuntimeGuidance()
+    {
+        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,
+            workspaceKind: "scratchpad",
+            projectInstructions: "Follow the repository guide.");
+
+        Assert.Contains("You are a helpful assistant.", instructions, StringComparison.Ordinal);
+        Assert.Contains("Follow the repository guide.", instructions, StringComparison.Ordinal);
+        Assert.Contains("scratchpad mode", instructions, StringComparison.OrdinalIgnoreCase);
+        Assert.True(
+            instructions.IndexOf("You are a helpful assistant.", StringComparison.Ordinal)
+            < instructions.IndexOf("Follow the repository guide.", StringComparison.Ordinal));
+        Assert.True(
+            instructions.IndexOf("Follow the repository guide.", StringComparison.Ordinal)
+            < instructions.IndexOf("scratchpad mode", StringComparison.OrdinalIgnoreCase));
+    }
+
     private static PatternAgentDefinitionDto CreateAgent(string id, string name, string instructions)
     {
         return new PatternAgentDefinitionDto

sidecar/tests/Aryx.AgentHost.Tests/CopilotAgentBundleTests.cs

@@ -250,6 +250,43 @@ public sealed class CopilotAgentBundleTests
         Assert.NotNull(sessionConfig.Hooks);
     }
 
+    [Fact]
+    public void CreateSessionConfig_PassesProjectInstructionsIntoTheSystemMessage()
+    {
+        RunTurnCommandDto command = new()
+        {
+            SessionId = "session-1",
+            ProjectPath = @"C:\workspace\project",
+            WorkspaceKind = "project",
+            Mode = "interactive",
+            ProjectInstructions = "Follow repository guidance.",
+            Pattern = new PatternDefinitionDto
+            {
+                Id = "pattern-1",
+                Name = "Pattern",
+                Mode = "single",
+                Availability = "available",
+                Agents =
+                [
+                    new PatternAgentDefinitionDto
+                    {
+                        Id = "agent-1",
+                        Name = "Primary",
+                        Model = "gpt-5.4",
+                        Instructions = "Help.",
+                    },
+                ],
+            },
+        };
+
+        SessionConfig sessionConfig = CopilotAgentBundle.CreateSessionConfig(
+            command,
+            command.Pattern.Agents[0],
+            agentIndex: 0);
+
+        Assert.Equal("Help.\n\nFollow repository guidance.", sessionConfig.SystemMessage?.Content);
+    }
+
     [Fact]
     public async Task CopilotSessionHooks_Create_UsesApprovalPolicyForPreToolUse()
     {