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/shared/domain/workspace.ts

@@ -10,6 +10,8 @@ export interface WorkspaceState {
   patterns: PatternDefinition[];
   sessions: SessionRecord[];
   settings: WorkspaceSettings;
+  /** IDs of built-in patterns the user has deleted. Prevents re-adding on load. */
+  deletedBuiltinPatternIds?: string[];
   /** Runtime-only MCP probe progress for live UI updates. */
   mcpProbingServerIds?: string[];
   selectedProjectId?: string;