fix(web): implement atomic write for shared storage

Add atomic write for the shared storage file to prevent race conditions.
Write to a temp file with restricted permissions and atomically rename.
Apply 600 permissions to the final storage file when possible

packages/web/package.json

@@ -25,6 +25,7 @@
     "@fontsource/ibm-plex-mono": "^5.2.7",
     "@fontsource/ibm-plex-sans": "^5.1.1",
     "@ibm/plex": "^6.4.1",
+    "@octokit/rest": "^22.0.1",
     "@opencode-ai/sdk": "^1.1.19",
     "@radix-ui/react-collapsible": "^1.1.12",
     "@radix-ui/react-dialog": "^1.1.15",