fix(server): Sandbox escape via arbitrary host path mounts in the default configuration

[Pangjiping]

Summary

• require explicit startup confirmation when api_key is unset
• Breaking change: empty or unset [storage].allowed_host_paths now denies all host mounts by default (instead of allowing all), and users who need legacy behavior must explicitly set allowed_host_paths = ["/"] as a temporary compatibility.
• closes SECURITY: Sandbox escape via arbitrary host path mounts in the default configuration. #750

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (allowed_host_paths = [] means allow all -> deny all)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[chatgpt-codex-connector]

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[hittyt]

LGTM