Shell mode silently drops floating attachments on submit

[Astro-Han]

Context

Found during the PR #1247 optimality review (Codex finding, verified). Pre-existing behavior — it already affected legacy data-URL image attachments before #1247; path-backed chips now inherit it.

Problem

createPromptSubmit lets a prompt with floating attachments (images or chips) pass the non-empty gate, but the shell branch only calls session.shell({ command: text }) — no parts — and then lifecycle.clearInput() wipes the composer (packages/app/src/components/prompt-input/submit.ts, shell branch).

Net effect: add an attachment (drag/paste, or restored via undo/fork/history), switch to shell mode, submit → the attachment disappears without being sent and without warning. The attach action in the menu is disabled in shell mode, but drag/paste/restore paths don't go through that button.

Possible directions

• Block shell submits while floating attachments exist (toast explaining why), or
• Preserve floating attachments in the composer across a shell submit instead of clearing them.

Either keeps the invariant: nothing visible in the composer is silently destroyed by a submit that does not send it.