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/components/attachment-chip.tsx

@@ -0,0 +1,27 @@
+import { Component } from "solid-js"
+import type { Attachment } from "../types/attachment"
+
+interface AttachmentChipProps {
+  attachment: Attachment
+  onRemove: () => void
+}
+
+const AttachmentChip: Component<AttachmentChipProps> = (props) => {
+  return (
+    <div
+      class="inline-flex items-center gap-1 rounded bg-blue-100 px-2 py-1 text-sm text-blue-800 dark:bg-blue-900/30 dark:text-blue-300"
+      title={props.attachment.source.type === "file" ? props.attachment.source.path : undefined}
+    >
+      <span class="font-mono">{props.attachment.display}</span>
+      <button
+        onClick={props.onRemove}
+        class="flex h-4 w-4 items-center justify-center rounded hover:bg-blue-200 dark:hover:bg-blue-800"
+        aria-label="Remove attachment"
+      >
+        ×
+      </button>
+    </div>
+  )
+}
+
+export default AttachmentChip