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/AgentInstructionComposer.cs

@@ -5,15 +5,15 @@ namespace Aryx.AgentHost.Services;
 internal static class AgentInstructionComposer
 {
     public static string Compose(
-        PatternDefinitionDto pattern,
-        PatternAgentDefinitionDto agent,
+        WorkflowDefinitionDto workflow,
+        WorkflowNodeDto agentNode,
         int agentIndex,
         string workspaceKind = "project",
         string interactionMode = "interactive",
         string? projectInstructions = null,
         RunTurnPromptInvocationDto? promptInvocation = null)
     {
-        string baseInstructions = agent.Instructions.Trim();
+        string baseInstructions = agentNode.Config.Instructions.Trim();
         string repositoryInstructions = projectInstructions?.Trim() ?? string.Empty;
         string promptInvocationInstructions = FormatPromptInvocation(promptInvocation);
         string workspaceGuidance = string.Equals(workspaceKind, "scratchpad", StringComparison.OrdinalIgnoreCase)
@@ -34,7 +34,7 @@ internal static class AgentInstructionComposer
               """
             : string.Empty;
 
-        if (string.Equals(pattern.Mode, "group-chat", StringComparison.OrdinalIgnoreCase))
+        if (string.Equals(workflow.Settings.OrchestrationMode, "group-chat", StringComparison.OrdinalIgnoreCase))
         {
             string groupChatGuidance = agentIndex == 0
                 ? """