feat: support project copilot customization

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

sidecar/src/Aryx.AgentHost/Contracts/ProtocolModels.cs

@@ -183,6 +183,7 @@ public sealed class RunTurnCommandDto : SidecarCommandEnvelope
     public string WorkspaceKind { get; init; } = "project";
     public string Mode { get; init; } = "interactive";
     public string MessageMode { get; init; } = "enqueue";
+    public string? ProjectInstructions { get; init; }
     public PatternDefinitionDto Pattern { get; init; } = new();
     public IReadOnlyList<ChatMessageDto> Messages { get; init; } = [];
     public RunTurnToolingConfigDto? Tooling { get; init; }

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

@@ -9,9 +9,11 @@ internal static class AgentInstructionComposer
         PatternAgentDefinitionDto agent,
         int agentIndex,
         string workspaceKind = "project",
-        string interactionMode = "interactive")
+        string interactionMode = "interactive",
+        string? projectInstructions = null)
     {
         string baseInstructions = agent.Instructions.Trim();
+        string repositoryInstructions = projectInstructions?.Trim() ?? string.Empty;
         string workspaceGuidance = string.Equals(workspaceKind, "scratchpad", StringComparison.OrdinalIgnoreCase)
             ? """
               You are operating in scratchpad mode.
@@ -46,12 +48,12 @@ internal static class AgentInstructionComposer
                   Focus on refining the answer already in progress.
                   """;
 
-            return JoinInstructionBlocks(baseInstructions, workspaceGuidance, planModeGuidance, groupChatGuidance);
+            return JoinInstructionBlocks(baseInstructions, repositoryInstructions, workspaceGuidance, planModeGuidance, groupChatGuidance);
         }
 
         if (!string.Equals(pattern.Mode, "handoff", StringComparison.OrdinalIgnoreCase))
         {
-            return JoinInstructionBlocks(baseInstructions, workspaceGuidance, planModeGuidance);
+            return JoinInstructionBlocks(baseInstructions, repositoryInstructions, workspaceGuidance, planModeGuidance);
         }
 
         string runtimeGuidance = agentIndex == 0
@@ -69,7 +71,7 @@ internal static class AgentInstructionComposer
               Do not push the actual work back to triage unless you are blocked or the request is clearly outside your specialty.
               """;
 
-        return JoinInstructionBlocks(baseInstructions, workspaceGuidance, planModeGuidance, runtimeGuidance);
+        return JoinInstructionBlocks(baseInstructions, repositoryInstructions, workspaceGuidance, planModeGuidance, runtimeGuidance);
     }
 
     private static string JoinInstructionBlocks(params string[] blocks)

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

@@ -103,7 +103,8 @@ internal sealed class CopilotAgentBundle : IAsyncDisposable
                     definition,
                     agentIndex,
                     command.WorkspaceKind,
-                    command.Mode),
+                    command.Mode,
+                    command.ProjectInstructions),
             },
             WorkingDirectory = command.ProjectPath,
             OnPermissionRequest = onPermissionRequest,

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()
     {