Add file attachments with @ mentions and drag & drop support

- Create attachment type system with file, text, symbol, and agent sources
- Implement file picker with SDK integration (find.files, file.status)
- Add @ detection in prompt input to trigger file search
- Create attachment chips UI for managing attached files
- Add attachment state management per session
- Update message submission to include attachments
- Implement drag & drop support for adding files
- Show git-modified files first in file picker with +/- indicators
- Filter files as user types after @
- Clear attachments after successful message send

src/stores/attachments.ts

@@ -0,0 +1,47 @@
+import { createSignal } from "solid-js"
+import type { Attachment } from "../types/attachment"
+
+const [attachments, setAttachments] = createSignal<Map<string, Attachment[]>>(new Map())
+
+function getSessionKey(instanceId: string, sessionId: string): string {
+  return `${instanceId}:${sessionId}`
+}
+
+function getAttachments(instanceId: string, sessionId: string): Attachment[] {
+  const key = getSessionKey(instanceId, sessionId)
+  return attachments().get(key) || []
+}
+
+function addAttachment(instanceId: string, sessionId: string, attachment: Attachment) {
+  const key = getSessionKey(instanceId, sessionId)
+  setAttachments((prev) => {
+    const next = new Map(prev)
+    const existing = next.get(key) || []
+    next.set(key, [...existing, attachment])
+    return next
+  })
+}
+
+function removeAttachment(instanceId: string, sessionId: string, attachmentId: string) {
+  const key = getSessionKey(instanceId, sessionId)
+  setAttachments((prev) => {
+    const next = new Map(prev)
+    const existing = next.get(key) || []
+    next.set(
+      key,
+      existing.filter((a) => a.id !== attachmentId),
+    )
+    return next
+  })
+}
+
+function clearAttachments(instanceId: string, sessionId: string) {
+  const key = getSessionKey(instanceId, sessionId)
+  setAttachments((prev) => {
+    const next = new Map(prev)
+    next.delete(key)
+    return next
+  })
+}
+
+export { getAttachments, addAttachment, removeAttachment, clearAttachments }