feat(editor): scaffold editor package

UI-only feature package for the ADR-029 editor rebuild: tsconfig
rootDir '.', vitest jsdom base with '@' alias and honest L0 baseline
thresholds, conformance ESLint active, React 19, @xyflow/react +
zustand wired against the pre-approved 2026-07-12 library traces.
Minimal feature.manifest.ts (empty useCases — no binders; declares
requiredCores runner-protocol + baseline coverage band) so the editor
participates in pnpm conformance without weakening any lint rule.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/editor/src/index.ts

@@ -0,0 +1,4 @@
+// Public contract surface of @repo/editor (types, schemas, manifest).
+// UI artifacts (board, frame node, overlay, store hook) live behind
+// `@repo/editor/ui` — see src/ui/index.ts.
+export { editorManifest, type EditorManifest } from "./feature.manifest";