editor: fix circular import that dropped PubScript nodes from schema

The TipTap node definitions imported their node-name constants from
schema.ts, while schema.ts imported the nodes to build the schema. This
circular dependency evaluated the nodes before the constants were
assigned, leaving sceneBreak/footnote/footnoteArea/characterMention with
name 'undefined' — so they never registered in PubScriptSchema. As a
result <hr class="scene-break"> parsed as StarterKit's horizontalRule
instead of a scene break.

- Move node-name constants, groups, MentionAttrs, and colorAttribute into
  a new constants.ts module that both schema.ts and the node files import
  from, breaking the cycle.
- Remove the redundant separate History extension (already in StarterKit),
  which also silenced a TipTap 'Duplicate extension names: undoRedo' warn.
- Drop unused TipTap extension and prosemirror-* deps (all access goes
  through @tiptap/pm).

packages/editor/package.json

@@ -0,0 +1,36 @@
+{
+  "name": "@pubscript/editor",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts",
+    "./schema": "./src/schema.ts",
+    "./Editor": "./src/Editor.tsx",
+    "./markdown": "./src/markdown.ts",
+    "./styles": "./src/styles/editor.css"
+  },
+  "scripts": {
+    "dev": "echo \"@pubscript/editor has no dev server\" (exit 0)",
+    "build": "tsc --noEmit",
+    "test": "vitest run",
+    "lint": "eslint .",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "@pubscript/core": "workspace:*",
+    "@tiptap/core": "3.32.0",
+    "@tiptap/pm": "3.32.0",
+    "@tiptap/react": "3.32.0",
+    "@tiptap/starter-kit": "3.32.0",
+    "@tiptap/extension-text-style": "3.32.0"
+  },
+  "devDependencies": {
+    "@types/node": "^22.10.2",
+    "@types/react": "^19.0.2",
+    "react": "^19.0.0",
+    "typescript": "^5.7.2",
+    "vitest": "^2.1.8"
+  }
+}