refactor: remove legacy patterns system, unify on workflows

Remove the entire patterns domain model, IPC channels, sidecar services,
renderer components, and tests. Sessions now bind exclusively to workflows
via workflowId. Builtin workflows replace builtin patterns.

Backend:
- Make AgentNodeConfig standalone (no longer extends PatternAgentDefinition)
- Add WorkflowOrchestrationMode and WorkflowExecutionDefinition
- Create builtin workflows (single-agent, sequential, concurrent, handoff, group-chat)
- Rewrite session model config helpers for workflow-only
- Remove pattern IPC channels, handlers, and preload bindings
- Merge createSession/createWorkflowSession into single method
- Remove sidecar PatternGraphResolver, PatternValidator, pattern DTOs
- Add workspace migration for legacy sessions (patternId -> workflowId)

Frontend:
- Delete PatternEditor, pattern-graph components, patternGraph lib
- Delete NewSessionModal (session creation uses workflows directly)
- Remove PatternsSection from SettingsPanel
- Update App.tsx, ChatPane, ActivityPanel, Sidebar, RunTimeline,
  AgentConfigFields, InlinePills, sessionActivity to use workflow types
- Delete pattern.ts domain module

78 files changed across backend and frontend.

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

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

@@ -186,54 +186,52 @@ public sealed class WorkflowRequestInfoInterpreterTests
     }
 
     private static RunTurnCommandDto CreateSingleAgentCommand()
-    {
-        return new RunTurnCommandDto
-        {
-            RequestId = "turn-1",
-            SessionId = "session-1",
-            Pattern = new PatternDefinitionDto
-            {
-                Id = "pattern-single",
-                Name = "Single Agent",
-                Mode = "single",
-                Availability = "available",
-                Agents =
-                [
-                    CreateAgent("agent-1", "Primary"),
-                ],
-            },
-        };
-    }
+        => CreateCommand("single", [CreateAgent("agent-1", "Primary")]);
 
     private static RunTurnCommandDto CreateHandoffCommand()
+        => CreateCommand("handoff",
+        [
+            CreateAgent("agent-handoff-triage", "Triage"),
+            CreateAgent("agent-handoff-ux", "UX Specialist"),
+        ]);
+
+    private static RunTurnCommandDto CreateCommand(string orchestrationMode, IReadOnlyList<WorkflowNodeDto> agents)
     {
         return new RunTurnCommandDto
         {
             RequestId = "turn-1",
             SessionId = "session-1",
-            Pattern = new PatternDefinitionDto
+            Workflow = new WorkflowDefinitionDto
             {
-                Id = "pattern-handoff",
-                Name = "Handoff Flow",
-                Mode = "handoff",
-                Availability = "available",
-                Agents =
-                [
-                    CreateAgent("agent-handoff-triage", "Triage"),
-                    CreateAgent("agent-handoff-ux", "UX Specialist"),
-                ],
+                Id = $"{orchestrationMode}-workflow",
+                Name = "Workflow",
+                Graph = new WorkflowGraphDto
+                {
+                    Nodes = [.. agents],
+                },
+                Settings = new WorkflowSettingsDto
+                {
+                    OrchestrationMode = orchestrationMode,
+                },
             },
         };
     }
 
-    private static PatternAgentDefinitionDto CreateAgent(string id, string name)
+    private static WorkflowNodeDto CreateAgent(string id, string name)
     {
-        return new PatternAgentDefinitionDto
+        return new WorkflowNodeDto
         {
             Id = id,
-            Name = name,
-            Model = "gpt-5.4",
-            Instructions = "Help with the request.",
+            Kind = "agent",
+            Label = name,
+            Config = new WorkflowNodeConfigDto
+            {
+                Kind = "agent",
+                Id = id,
+                Name = name,
+                Model = "gpt-5.4",
+                Instructions = "Help with the request.",
+            },
         };
     }