feat: allow deletion of built-in orchestration patterns

Remove the restriction that prevented users from deleting built-in
patterns (those with IDs prefixed 'pattern-'). Built-in patterns are
intended as starting points, not permanent fixtures.

Changes:
- Add deletedBuiltinPatternIds field to WorkspaceState to track which
  built-in patterns the user has removed
- Update mergePatterns to skip deleted built-ins during workspace load,
  preventing them from being re-added
- Remove the isBuiltinPattern guard from AryxAppService.deletePattern;
  when a built-in is deleted, its ID is recorded in the tracking list
- Enable the delete button in SettingsPanel and PatternEditor for all
  patterns (keep the 'Built-in pattern' label for context)
- Add 3 tests covering built-in deletion, custom deletion, and
  selectedPatternId fallback

Users can still restore deleted built-ins via workspace reset.

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

src/main/AryxAppService.ts

@@ -710,13 +710,15 @@ export class AryxAppService extends EventEmitter<AppServiceEvents> {
   }
 
   async deletePattern(patternId: string): Promise<WorkspaceState> {
-    if (isBuiltinPattern(patternId)) {
-      throw new Error('Built-in patterns cannot be deleted.');
-    }
-
     const workspace = await this.loadWorkspace();
     workspace.patterns = workspace.patterns.filter((pattern) => pattern.id !== patternId);
 
+    if (isBuiltinPattern(patternId)) {
+      const deletedIds = new Set(workspace.deletedBuiltinPatternIds ?? []);
+      deletedIds.add(patternId);
+      workspace.deletedBuiltinPatternIds = [...deletedIds];
+    }
+
     if (workspace.selectedPatternId === patternId) {
       workspace.selectedPatternId = workspace.patterns[0]?.id;
     }

src/main/persistence/workspaceRepository.ts

@@ -30,13 +30,16 @@ import {
 } from '@main/persistence/appPaths';
 import { readJsonFile, writeJsonFile } from '@main/persistence/jsonStore';
 
-function mergePatterns(existingPatterns: PatternDefinition[]): PatternDefinition[] {
+function mergePatterns(existingPatterns: PatternDefinition[], deletedBuiltinIds: string[]): PatternDefinition[] {
   const builtinTimestamp = nowIso();
   const builtinPatterns = createBuiltinPatterns(builtinTimestamp);
   const builtinIds = new Set(builtinPatterns.map((pattern) => pattern.id));
+  const deletedSet = new Set(deletedBuiltinIds);
   const existingMap = new Map(existingPatterns.map((pattern) => [pattern.id, pattern]));
 
-  const mergedBuiltins = builtinPatterns.map((builtin) => {
+  const mergedBuiltins = builtinPatterns
+    .filter((builtin) => !deletedSet.has(builtin.id))
+    .map((builtin) => {
     const existing = existingMap.get(builtin.id);
     if (!existing) {
       return builtin;
@@ -108,9 +111,11 @@ export class WorkspaceRepository {
     }));
     const settings = normalizeWorkspaceSettings(stored.settings);
 
+    const deletedBuiltinPatternIds = stored.deletedBuiltinPatternIds ?? [];
+
     const workspace: WorkspaceState = {
       ...stored,
-      patterns: mergePatterns(stored.patterns ?? []).map((pattern) => ({
+      patterns: mergePatterns(stored.patterns ?? [], deletedBuiltinPatternIds).map((pattern) => ({
         ...pattern,
         approvalPolicy: applyDefaultToolApprovalPolicy(pattern.approvalPolicy),
         graph: resolvePatternGraph(pattern),
@@ -118,6 +123,7 @@ export class WorkspaceRepository {
       projects,
       sessions,
       settings,
+      deletedBuiltinPatternIds,
       selectedProjectId: projects.some((project) => project.id === stored.selectedProjectId)
         ? stored.selectedProjectId
         : projects[0]?.id,