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/src/Aryx.AgentHost/Services/WorkflowDefinitionExtensions.cs

@@ -0,0 +1,54 @@
+using Aryx.AgentHost.Contracts;
+
+namespace Aryx.AgentHost.Services;
+
+internal static class WorkflowDefinitionExtensions
+{
+    public static IReadOnlyList<WorkflowNodeDto> GetAgentNodes(this WorkflowDefinitionDto workflow)
+    {
+        ArgumentNullException.ThrowIfNull(workflow);
+
+        return workflow.Graph.Nodes
+            .Where(IsAgentNode)
+            .ToList();
+    }
+
+    public static bool IsAgentNode(this WorkflowNodeDto node)
+    {
+        ArgumentNullException.ThrowIfNull(node);
+        return string.Equals(node.Kind, "agent", StringComparison.OrdinalIgnoreCase);
+    }
+
+    public static string GetAgentId(this WorkflowNodeDto node)
+    {
+        ArgumentNullException.ThrowIfNull(node);
+        return !string.IsNullOrWhiteSpace(node.Config.Id) ? node.Config.Id : node.Id;
+    }
+
+    public static string GetAgentName(this WorkflowNodeDto node)
+    {
+        ArgumentNullException.ThrowIfNull(node);
+        return FirstNonBlank(node.Config.Name, node.Label, node.Id) ?? "agent";
+    }
+
+    public static bool IsOrchestrationMode(this WorkflowDefinitionDto workflow, string mode)
+    {
+        ArgumentNullException.ThrowIfNull(workflow);
+        ArgumentException.ThrowIfNullOrWhiteSpace(mode);
+
+        return string.Equals(workflow.Settings.OrchestrationMode, mode, StringComparison.OrdinalIgnoreCase);
+    }
+
+    private static string? FirstNonBlank(params string?[] values)
+    {
+        foreach (string? value in values)
+        {
+            if (!string.IsNullOrWhiteSpace(value))
+            {
+                return value.Trim();
+            }
+        }
+
+        return null;
+    }
+}