[Gap]: Refresh playground.mdx for thread management and file attachments

[danbarr]

Gap

Two sets of playground features are undocumented:

1. Thread management (sidebar): create / rename / delete / star-unstar threads, sorted by last edited. Added in feat(playground): threaded chat with sidebar, LLM titles, star/rename/delete, and scroll persistence toolhive-studio#1915.
2. File attachments: prompt input supports file attachments via PromptInputActionAddAttachments, with an AttachmentPreview component. Added in feat(playground): restyle and add possibility to attach files toolhive-studio#913.

playground.mdx doesn't mention either — it describes only "begin chatting" as the interaction model.

Source in app (v0.30.0)

• renderer/src/features/chat/components/playground-sidebar.tsx
• renderer/src/features/chat/components/thread-title-bar.tsx
• renderer/src/features/chat/components/chat-input-prompt.tsx
• renderer/src/features/chat/components/attachment-preview.tsx

Docs to update

• docs/toolhive/guides-ui/playground.mdx

Introducing PRs

• feat(playground): threaded chat with sidebar, LLM titles, star/rename/delete, and scroll persistence toolhive-studio#1915 — feat(playground): threaded chat with sidebar, LLM titles, star/rename/delete, and scroll persistence
• feat(playground): restyle and add possibility to attach files toolhive-studio#913 — feat(playground): restyle and add possibility to attach files

Parent

Part of #751.