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/WorkflowRunner.cs

@@ -8,12 +8,10 @@ internal sealed class WorkflowRunner
 {
     public Workflow BuildWorkflow(
         WorkflowDefinitionDto workflowDefinition,
-        PatternDefinitionDto patternDefinition,
         IReadOnlyList<AIAgent> agents,
         IReadOnlyList<WorkflowDefinitionDto>? workflowLibrary = null)
     {
         ArgumentNullException.ThrowIfNull(workflowDefinition);
-        ArgumentNullException.ThrowIfNull(patternDefinition);
         ArgumentNullException.ThrowIfNull(agents);
 
         Dictionary<string, WorkflowDefinitionDto> workflowLibraryMap = workflowLibrary?
@@ -22,9 +20,10 @@ internal sealed class WorkflowRunner
             .ToDictionary(group => group.Key, group => group.Last(), StringComparer.Ordinal)
             ?? new Dictionary<string, WorkflowDefinitionDto>(StringComparer.Ordinal);
 
-        Dictionary<string, AIAgent> agentMap = patternDefinition.Agents
-            .Zip(agents, (definition, agent) => (definition.Id, agent))
-            .ToDictionary(pair => pair.Id, pair => pair.agent, StringComparer.Ordinal);
+        List<string> agentIds = ResolveAgentIds(workflowDefinition, workflowLibraryMap);
+        Dictionary<string, AIAgent> agentMap = agentIds
+            .Zip(agents, (agentId, agent) => (agentId, agent))
+            .ToDictionary(pair => pair.agentId, pair => pair.agent, StringComparer.Ordinal);
 
         return BuildWorkflow(workflowDefinition, agentMap, workflowLibraryMap);
     }
@@ -222,6 +221,47 @@ internal sealed class WorkflowRunner
     private static string? NormalizeOptionalString(string? value)
         => string.IsNullOrWhiteSpace(value) ? null : value.Trim();
 
+    private static List<string> ResolveAgentIds(
+        WorkflowDefinitionDto workflowDefinition,
+        IReadOnlyDictionary<string, WorkflowDefinitionDto> workflowLibrary)
+    {
+        List<string> agentIds = [];
+        CollectAgentIds(workflowDefinition, workflowLibrary, agentIds, new HashSet<string>(StringComparer.Ordinal));
+        return agentIds;
+    }
+
+    private static void CollectAgentIds(
+        WorkflowDefinitionDto workflowDefinition,
+        IReadOnlyDictionary<string, WorkflowDefinitionDto> workflowLibrary,
+        List<string> agentIds,
+        ISet<string> visitedWorkflowIds)
+    {
+        string workflowKey = string.IsNullOrWhiteSpace(workflowDefinition.Id)
+            ? Guid.NewGuid().ToString("N")
+            : workflowDefinition.Id;
+        if (!visitedWorkflowIds.Add(workflowKey))
+        {
+            return;
+        }
+
+        foreach (WorkflowNodeDto node in workflowDefinition.Graph.Nodes)
+        {
+            if (string.Equals(node.Kind, "agent", StringComparison.OrdinalIgnoreCase))
+            {
+                agentIds.Add(!string.IsNullOrWhiteSpace(node.Config.Id) ? node.Config.Id : node.Id);
+                continue;
+            }
+
+            if (!string.Equals(node.Kind, "sub-workflow", StringComparison.OrdinalIgnoreCase))
+            {
+                continue;
+            }
+
+            WorkflowDefinitionDto subWorkflow = ResolveSubWorkflowDefinition(node, workflowLibrary);
+            CollectAgentIds(subWorkflow, workflowLibrary, agentIds, visitedWorkflowIds);
+        }
+    }
+
     private sealed record WorkflowNodeRoute(
         ExecutorBinding Entry,
         ExecutorBinding Exit,