refactor(ui): split prompt input into hooks and API

Extract prompt draft/history, attachments, picker, and keydown logic into co-located hooks. Introduce PromptInputApi for quote/expand/setText and migrate SessionView off DOM poking; remove legacy registerQuoteHandler.

packages/ui/src/components/prompt-input/PromptAttachmentsBar.tsx

@@ -0,0 +1,50 @@
+import { For, Show, type Component } from "solid-js"
+import { Expand } from "lucide-solid"
+import type { Attachment } from "../../types/attachment"
+import { useI18n } from "../../lib/i18n"
+
+interface PromptAttachmentsBarProps {
+  attachments: Attachment[]
+  onRemoveAttachment: (attachmentId: string) => void
+  onExpandTextAttachment: (attachmentId: string) => void
+}
+
+const PromptAttachmentsBar: Component<PromptAttachmentsBarProps> = (props) => {
+  const { t } = useI18n()
+
+  return (
+    <div class="flex flex-wrap items-center gap-1.5 border-t px-3 py-2" style="border-color: var(--border-base);">
+      <For each={props.attachments}>
+        {(attachment) => {
+          const isText = attachment.source.type === "text"
+          return (
+            <div class="attachment-chip" title={attachment.source.type === "file" ? attachment.source.path : undefined}>
+              <span class="font-mono">{attachment.display}</span>
+              <Show when={isText}>
+                <button
+                  type="button"
+                  class="attachment-expand"
+                  onClick={() => props.onExpandTextAttachment(attachment.id)}
+                  aria-label={t("sessionView.attachments.expandPastedTextAriaLabel")}
+                  title={t("sessionView.attachments.insertPastedTextTitle")}
+                >
+                  <Expand class="h-3 w-3" aria-hidden="true" />
+                </button>
+              </Show>
+              <button
+                type="button"
+                class="attachment-remove"
+                onClick={() => props.onRemoveAttachment(attachment.id)}
+                aria-label={t("sessionView.attachments.removeAriaLabel")}
+              >
+                ×
+              </button>
+            </div>
+          )
+        }}
+      </For>
+    </div>
+  )
+}
+
+export default PromptAttachmentsBar