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/types/attachment.ts

@@ -0,0 +1,87 @@
+export interface Attachment {
+  id: string
+  type: AttachmentType
+  display: string
+  url: string
+  filename: string
+  mediaType: string
+  source: AttachmentSource
+}
+
+export type AttachmentType = "file" | "text" | "symbol" | "agent"
+
+export type AttachmentSource = FileSource | TextSource | SymbolSource | AgentSource
+
+export interface FileSource {
+  type: "file"
+  path: string
+  mime: string
+  data?: Uint8Array
+}
+
+export interface TextSource {
+  type: "text"
+  value: string
+}
+
+export interface SymbolSource {
+  type: "symbol"
+  path: string
+  name: string
+  kind: number
+  range: SymbolRange
+}
+
+export interface SymbolRange {
+  start: Position
+  end: Position
+}
+
+export interface Position {
+  line: number
+  char: number
+}
+
+export interface AgentSource {
+  type: "agent"
+  name: string
+}
+
+export function createFileAttachment(
+  path: string,
+  filename: string,
+  mime: string = "text/plain",
+  data?: Uint8Array,
+): Attachment {
+  const absolutePath = path.startsWith("/") ? path : `/${path}`
+  return {
+    id: crypto.randomUUID(),
+    type: "file",
+    display: `@${filename}`,
+    url: `file://${absolutePath}`,
+    filename,
+    mediaType: mime,
+    source: {
+      type: "file",
+      path: absolutePath,
+      mime,
+      data,
+    },
+  }
+}
+
+export function createTextAttachment(value: string, display: string, filename: string): Attachment {
+  const base64 = btoa(value)
+  return {
+    id: crypto.randomUUID(),
+    type: "text",
+    display,
+    url: `data:text/plain;base64,${base64}`,
+    filename,
+    mediaType: "text/plain",
+    source: {
+      type: "text",
+      value,
+    },
+  }
+}