feat(workflows): Phase 1 backend — WorkflowDefinition model, persistence, IPC, sidecar execution

Introduce WorkflowDefinition as a first-class entity alongside PatternDefinition.

Shared domain:
- src/shared/domain/workflow.ts: new WorkflowDefinition, WorkflowGraph,
  WorkflowNode/Edge/Config types, normalization, TS-side validation, and
  buildWorkflowExecutionPattern (synthetic pattern bridge for session plumbing)
- workspace.ts: add workflows[] and selectedWorkflowId
- session.ts: add optional workflowId to SessionRecord
- runTimeline.ts: add optional workflowId/workflowName to run records
- sessionLibrary.ts: workflow-aware session search

IPC / main process:
- contracts/channels.ts, contracts/ipc.ts: workflow CRUD channels
- preload/index.ts: expose saveWorkflow, deleteWorkflow, createWorkflowSession
- ipc/registerIpcHandlers.ts: register workflow handlers
- persistence/workspaceRepository.ts: load/save/normalize workflows
- AryxAppService.ts: saveWorkflow, deleteWorkflow, createWorkflowSession,
  resolveSessionExecutionDefinition, workflow-aware turn/run plumbing

Sidecar protocol:
- contracts/sidecar.ts: ValidateWorkflowCommand, WorkflowValidationEvent,
  optional workflow on RunTurnCommand
- sidecarProcess.ts: validate-workflow command dispatch and event handling
- Contracts/ProtocolModels.cs: workflow DTOs and validate-workflow DTOs
- Services/SidecarProtocolHost.cs: validate-workflow command handler
- Services/CopilotWorkflowRunner.cs: workflow-aware build and checkpoint path
- Services/WorkflowValidator.cs: graph validation (connectivity, start/end,
  fan-out/in arity, path reachability)
- Services/WorkflowRunner.cs: WorkflowDefinitionDto -> Agent Framework Workflow
  builder (direct, fan-out, fan-in edges; agent executor binding)

Project: bump both csproj from net9.0 to net10.0

Tests:
- tests/shared/workflow.test.ts: TS workflow validation and pattern synthesis
- tests/main/appServiceWorkflow.test.ts: saveWorkflow / createWorkflowSession
- tests/Aryx.AgentHost.Tests/SidecarProtocolHostTests.cs: ValidateWorkflowCommand
- Fix two stale workspace fixtures missing workflows field

Validation: tsc clean, bun test 367/367 pass, dotnet test 244/244 pass, bun run build succeeds.

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

sidecar/tests/Aryx.AgentHost.Tests/Aryx.AgentHost.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net9.0</TargetFramework>
+    <TargetFramework>net10.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>

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

@@ -108,6 +108,76 @@ public sealed class SidecarProtocolHostTests
             });
     }
 
+    [Fact]
+    public async Task ValidateWorkflowCommand_ReturnsIssuesAndCompletion()
+    {
+        IReadOnlyList<JsonElement> events = await RunHostAsync(new ValidateWorkflowCommandDto
+        {
+            Type = "validate-workflow",
+            RequestId = "validate-workflow-1",
+            Workflow = new WorkflowDefinitionDto
+            {
+                Id = "workflow-1",
+                Name = "",
+                Graph = new WorkflowGraphDto
+                {
+                    Nodes =
+                    [
+                        new WorkflowNodeDto
+                        {
+                            Id = "start",
+                            Kind = "start",
+                            Label = "Start",
+                            Config = new WorkflowNodeConfigDto { Kind = "start" },
+                        },
+                        new WorkflowNodeDto
+                        {
+                            Id = "end",
+                            Kind = "end",
+                            Label = "End",
+                            Config = new WorkflowNodeConfigDto { Kind = "end" },
+                        },
+                    ],
+                    Edges =
+                    [
+                        new WorkflowEdgeDto
+                        {
+                            Id = "edge-start-end",
+                            Source = "start",
+                            Target = "end",
+                            Kind = "direct",
+                        },
+                    ],
+                },
+                Settings = new WorkflowSettingsDto
+                {
+                    Checkpointing = new WorkflowCheckpointSettingsDto(),
+                },
+            },
+        });
+
+        Assert.Collection(
+            events,
+            validationEvent =>
+            {
+                Assert.Equal("workflow-validation", validationEvent.GetProperty("type").GetString());
+                Assert.Equal("validate-workflow-1", validationEvent.GetProperty("requestId").GetString());
+
+                JsonElement[] issues = validationEvent.GetProperty("issues").EnumerateArray().ToArray();
+                Assert.Contains(issues, issue =>
+                    issue.GetProperty("field").GetString() == "name"
+                    && issue.GetProperty("message").GetString() == "Workflow name is required.");
+                Assert.Contains(issues, issue =>
+                    issue.GetProperty("field").GetString() == "graph.nodes"
+                    && issue.GetProperty("message").GetString() == "Workflow graphs must contain at least one agent node.");
+            },
+            completionEvent =>
+            {
+                Assert.Equal("command-complete", completionEvent.GetProperty("type").GetString());
+                Assert.Equal("validate-workflow-1", completionEvent.GetProperty("requestId").GetString());
+            });
+    }
+
     [Fact]
     public async Task RunTurnCommand_ReturnsActivityEventsAndCompletion()
     {