Phase 14: multi-host file browser (0.15.0)

The Files page gained a host switcher: when agents are registered, a Host
dropdown switches the whole browser between the local host and any online agent
(switching resets path + clipboard). Every file operation is sandboxed by the
selected agent's own ALLOWED_BROWSE_ROOTS/HOST_ROOT_PREFIX.

- agent_app.py: /agent/files/* (list/read/download/write/mkdir/touch/rename/
  copy/move/delete/upload) reusing file_service + device_service; BrowseError
  -> HTTP 400.
- routers/agents.py: proxy routes at /api/agents/{id}/files/* (audit-logged
  mutations); download streams via download_to_file, upload via upload_file.
  Reuses the WriteBody/NameBody/RenameBody/TransferBody models from routers.files.
- Frontend: filesApi methods take an optional trailing agentId; Files.tsx tracks
  a host and threads it through every call, query key, and the editor/dialogs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.14.0",
+  "version": "0.15.0",
   "type": "module",
   "scripts": {
     "dev": "vite",