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/attachmentPlaceholders.ts

@@ -0,0 +1,72 @@
+import type { Attachment } from "../../types/attachment"
+
+export function formatPastedPlaceholder(value: string | number) {
+  return `[pasted #${value}]`
+}
+
+export function formatImagePlaceholder(value: string | number) {
+  return `[Image #${value}]`
+}
+
+export function createPastedPlaceholderRegex() {
+  return /\[pasted #(\d+)\]/g
+}
+
+export function createImagePlaceholderRegex() {
+  return /\[Image #(\d+)\]/g
+}
+
+export function createMentionRegex() {
+  return /@(\S+)/g
+}
+
+export const pastedDisplayCounterRegex = /pasted #(\d+)/
+export const imageDisplayCounterRegex = /Image #(\d+)/
+export const bracketedImageDisplayCounterRegex = /\[Image #(\d+)\]/
+
+export function parseCounter(value: string) {
+  const parsed = Number.parseInt(value, 10)
+  return Number.isNaN(parsed) ? null : parsed
+}
+
+export function findHighestAttachmentCounters(currentPrompt: string, sessionAttachments: Attachment[]) {
+  let highestPaste = 0
+  let highestImage = 0
+
+  for (const match of currentPrompt.matchAll(createPastedPlaceholderRegex())) {
+    const parsed = parseCounter(match[1])
+    if (parsed !== null) {
+      highestPaste = Math.max(highestPaste, parsed)
+    }
+  }
+
+  for (const attachment of sessionAttachments) {
+    if (attachment.source.type === "text") {
+      const placeholderMatch = attachment.display.match(pastedDisplayCounterRegex)
+      if (placeholderMatch) {
+        const parsed = parseCounter(placeholderMatch[1])
+        if (parsed !== null) {
+          highestPaste = Math.max(highestPaste, parsed)
+        }
+      }
+    }
+    if (attachment.source.type === "file" && attachment.mediaType.startsWith("image/")) {
+      const imageMatch = attachment.display.match(imageDisplayCounterRegex)
+      if (imageMatch) {
+        const parsed = parseCounter(imageMatch[1])
+        if (parsed !== null) {
+          highestImage = Math.max(highestImage, parsed)
+        }
+      }
+    }
+  }
+
+  for (const match of currentPrompt.matchAll(createImagePlaceholderRegex())) {
+    const parsed = parseCounter(match[1])
+    if (parsed !== null) {
+      highestImage = Math.max(highestImage, parsed)
+    }
+  }
+
+  return { highestPaste, highestImage }
+}