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>

tests/shared/sessionLibrary.test.ts

@@ -73,9 +73,10 @@ function createSession(overrides?: Partial<SessionRecord>): SessionRecord {
 }
 
 function createWorkspace(overrides?: Partial<WorkspaceState>): WorkspaceState {
-  return {
+  const workspace: WorkspaceState = {
     projects: [createProject(), createProject({ id: 'project-scratchpad', name: 'Scratchpad', path: 'C:\\scratchpad' })],
     patterns: [createPattern(), createPattern({ id: 'pattern-single-chat', name: '1-on-1 Copilot Chat', mode: 'single' })],
+    workflows: [],
     settings: createWorkspaceSettings(),
     sessions: [
       createSession(),
@@ -112,6 +113,11 @@ function createWorkspace(overrides?: Partial<WorkspaceState>): WorkspaceState {
     lastUpdatedAt: '2026-03-23T00:10:00.000Z',
     ...overrides,
   };
+
+  return {
+    ...workspace,
+    workflows: overrides?.workflows ?? workspace.workflows,
+  };
 }
 
 describe('session library helpers', () => {