Opening the @ context menu reorders existing mentions and rewrites the draft

Description

On the session page with the v2 prompt composer, opening the @ context menu (the + button → "Context") silently rewrites the draft: any existing @ mentions get moved to the end of the prompt and the text between them loses its position.

Steps to reproduce

1. Open a session (the bug is in the v2 composer on the session page).
2. Type hi , then @src, and click the src/index.ts suggestion to insert a mention.
3. Type and, then @src again, and click the suggestion to insert a second mention.
4. The draft now reads: hi @src/index.ts and @src/index.ts.
5. Click the + button and select Context (or press Esc to close the menu without picking anything).

Expected behavior

The draft stays intact; the two mentions keep their positions and and remains between them.

Actual behavior

The draft is rewritten to something like hi and @@src/index.ts@src/index.ts — both mention chips are moved to the end, extra spaces appear, and the text between the mentions is displaced. This happens even if you immediately close the menu.

Root cause

openContext in packages/session-ui/src/v2/components/prompt-input/machine.ts unconditionally runs draft.setText(promptText(persisted) + "@"). promptText only joins text parts (dropping mention content), and setText moves every non-text part to the end of the prompt, so the mention structure is destroyed.