Remote MCP server OAuth flow fails inside `sbx` — no keyring directory available

[maxcleme]

Description

When using a remote MCP server (e.g. Notion at mcp.notion.com) from inside sbx, the OAuth2 popup triggers correctly but the flow fails when attempting to store the token, because no file keyring directory is available in the sandbox environment.

Error

Steps to Reproduce

1. Configure a remote MCP server

     - type: mcp
        remote:
          url: https://mcp.notion.com/sse
          transport_type: sse

2. Run docker-agent inside sbx
3. Observe the OAuth2 popup appear
4. Complete the OAuth flow
5. Agent fails to initialize — token cannot be stored

Expected Behavior

OAuth tokens should be stored in a location accessible from within the sandbox, or docker-agent should provide a fallback storage mechanism when a file keyring is unavailable.