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>

tests/shared/workflow.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, test } from 'bun:test';
 
 import {
-  buildWorkflowExecutionPattern,
+  buildWorkflowExecutionDefinition,
   validateWorkflowDefinition,
   type WorkflowDefinition,
 } from '@shared/domain/workflow';
@@ -245,16 +245,16 @@ describe('workflow validation', () => {
     expect(issues.some((issue) => issue.message.includes('exactly one of workflowId or inlineWorkflow'))).toBe(true);
   });
 
-  test('builds a synthetic execution pattern from workflow agents', () => {
-    const pattern = buildWorkflowExecutionPattern(createWorkflow());
+  test('builds an execution definition from workflow agents', () => {
+    const execution = buildWorkflowExecutionDefinition(createWorkflow());
 
-    expect(pattern.id).toBe('workflow-1');
-    expect(pattern.agents).toHaveLength(1);
-    expect(pattern.agents[0]?.name).toBe('Primary Agent');
-    expect(pattern.graph?.nodes.map((node) => node.kind)).toEqual(['user-input', 'agent', 'user-output']);
+    expect(execution.id).toBe('workflow-1');
+    expect(execution.agents).toHaveLength(1);
+    expect(execution.agents[0]?.name).toBe('Primary Agent');
+    expect(execution.orchestrationMode).toBe('single');
   });
 
-  test('includes referenced sub-workflow agents when building execution patterns', () => {
+  test('includes referenced sub-workflow agents when building execution definitions', () => {
     const childWorkflow = createReferencedSubWorkflow();
     const workflow = createWorkflow();
     workflow.graph.nodes[1] = {
@@ -270,12 +270,12 @@ describe('workflow validation', () => {
     workflow.graph.edges[0] = { id: 'edge-start-sub', source: 'start', target: 'sub-workflow', kind: 'direct' };
     workflow.graph.edges[1] = { id: 'edge-sub-end', source: 'sub-workflow', target: 'end', kind: 'direct' };
 
-    const pattern = buildWorkflowExecutionPattern(workflow, {
-      resolveWorkflow: (workflowId) => workflowId === childWorkflow.id ? childWorkflow : undefined,
+    const execution = buildWorkflowExecutionDefinition(workflow, {
+      resolveWorkflow: (workflowId: string) => workflowId === childWorkflow.id ? childWorkflow : undefined,
     });
 
-    expect(pattern.mode).toBe('single');
-    expect(pattern.agents.map((agent) => agent.id)).toEqual(['agent-reviewer']);
+    expect(execution.orchestrationMode).toBe('single');
+    expect(execution.agents.map((agent) => agent.id)).toEqual(['agent-reviewer']);
   });
 
   test('accepts simple property and expression conditions', () => {