arch: move Tiptap UI out of vendored packages/ui/src/components/

[cursor]

Problem

ADR 0001 and ui-components.md state that packages/ui/src/components/* is vendored from the coss registry and local edits are discarded on refresh. Durable product UI belongs in ai-elements/ or feature wrappers (exceptions: carousel, splitter).

The entire Tiptap subtree is hand-written application code sitting under the vendored tree:

• packages/ui/src/components/tiptap/suggestion-menu.tsx
• packages/ui/src/components/tiptap/suggestion-list.tsx
• packages/ui/src/components/tiptap/suggestion-menu-utils.ts
• packages/ui/src/components/tiptap/suggestion-menu-types.ts
• packages/ui/src/hooks/tiptap/use-floating-element.ts
• packages/ui/src/hooks/tiptap/use-menu-navigation.ts
• packages/ui/src/hooks/tiptap/use-tiptap-editor.ts

Why it matters

The next shadcn add @coss/ui --overwrite can wipe or conflict with this code. It also blurs the vendored vs. owned boundary the ADR exists to enforce.

Suggested fix

Move Tiptap UI to packages/ui/src/ai-elements/tiptap/ (or apps/app/src/features/chat/components/input/ if chat-only). Keep components/ strictly vendored.

Severity

P2