feat(react): add attach file slash command

[maccman]

Problem

Hosts can persist pasted or dropped files through onFilePaste, but users who prefer command menus had no built-in slash command for choosing a file from disk. The slash menu also needed to keep using the same markdown shape as paste/drop so attachments render consistently as images or file pills.

Before -> After

Before	After
/ showed the built-in formatting/table/time commands plus host items.	When onFilePaste is configured, /attach shows Attach file and opens a hidden file picker.
Hosts had to add their own slash item to persist selected files.	The React slash menu reuses onFilePaste, onFileSaveError, and buildFileMarkdown.

Changes

1. Added an optional Attach file slash menu item in SlashMenu when onFilePaste is present.
2. Added hidden multi-file input handling that persists files sequentially, formats each result with buildFileMarkdown, and inserts the markdown at the current editor selection.
3. Threaded onFilePaste and onFileSaveError from ProseKitEditor into SlashMenu.
4. Covered the attach flow in slash-menu.test.tsx, including query removal before insertion.

Verification

• git diff --check passed.
• pnpm vitest run packages/react/src/components/slash-menu.test.tsx passed.
• pnpm exec oxfmt --check packages/react/src/components/slash-menu.tsx packages/react/src/components/prosekit-editor.tsx packages/react/src/components/slash-menu.test.tsx passed.
• pnpm exec eslint --max-warnings 0 packages/react/src/components/slash-menu.tsx packages/react/src/components/prosekit-editor.tsx packages/react/src/components/slash-menu.test.tsx passed.
• pnpm typecheck passed.

Risk / Rollout

No migrations or rollout steps. The command is only shown when a host has already opted into file persistence via onFilePaste; hosts without that handler keep the existing menu behavior.

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npm i https://pkg.pr.new/prosekit/meowdown/@meowdown/core@261

npm i https://pkg.pr.new/prosekit/meowdown/@meowdown/react@261

commit: e5908c0

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Project	Deployment	Actions	Updated (UTC)
meowdown	Ready	Preview, Comment	Jul 8, 2026 9:40am