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/App.tsx

@@ -1,5 +1,6 @@
 import { Component, onMount, onCleanup, Show, createMemo, createEffect } from "solid-js"
 import type { Session } from "./types/session"
+import type { Attachment } from "./types/attachment"
 import EmptyState from "./components/empty-state"
 import SessionPicker from "./components/session-picker"
 import CommandPalette from "./components/command-palette"
@@ -70,8 +71,8 @@ const SessionView: Component<{
     }
   })
 
-  async function handleSendMessage(prompt: string) {
-    await sendMessage(props.instanceId, props.sessionId, prompt)
+  async function handleSendMessage(prompt: string, attachments: Attachment[]) {
+    await sendMessage(props.instanceId, props.sessionId, prompt, attachments)
   }
 
   async function handleAgentChange(agent: string) {