Studio catalog prompt editor has no accessible name #4384

The catalog's Ask agent dialog contains a prompt editor with no accessible name. Chrome exposes the dialog title, but its textbox is unnamed, so assistive technology cannot identify the field independently of surrounding text.

Reproduction and evidence

Verified against origin/main 86fa10c on September 23, 2026. The relevant component is unchanged from the source used in the browser probe.

1. Open Studio's catalog and use a block card's Ask agent action.
2. Inspect the prompt textarea in Chrome's accessibility tree.
3. The textbox's computed name is empty; the dialog itself is named Ask agent — <block title>.

An isolated browser mount of the actual PromptPreviewModal with title Example and prompt Change the title produced:

[
  { "role": "dialog", "name": "Ask agent — Example" },
  { "role": "textbox", "name": "", "value": "Change the title" }
]

This was verified through Chrome's accessibility tree, not a screen-reader session or full Studio end-to-end run. The catalog caller is source-verified.

Scope

Give this prompt editor a stable, meaningful accessible name using a label or appropriate ARIA association. Preserve the existing prompt text, dialog title, editing, copy action, and keyboard shortcut. No modal redesign is needed.

The separate selection AskAgentModal already receives a placeholder-derived accessible name in Chrome and is outside this issue's scope.

Acceptance criteria

• The prompt textbox has a meaningful nonempty accessible name, including when the prompt is populated or edited.
• The field name is distinct from the editor's current value and the dialog title.
• Copying the edited prompt and the existing Ctrl/Cmd+Enter shortcut still work.
• A focused component test checks the textbox's accessible name; a browser accessibility inspection confirms the result.

Code pointers and validation

• PromptPreviewModal textarea
• Catalog dialog caller
• Read packages/studio/AGENTS.md; run Studio tests with Vitest through bun run --cwd packages/studio test, not bare bun test.

Difficulty: easy — a localized labeling correction with direct accessibility verification. Before starting, check current assignments and linked PRs. Open UI migration PRs #3625, #3626, #3774 and #3776 contain only CSS-class changes in this component; their inspected patches do not add its field label.

Reviewer/mentor is being arranged. Comment with your intended approach before starting so ownership can be coordinated; this issue is not yet a newcomer invitation.