Check Drive export permission before file and document media downloads, while preserving whiteboard behavior and adding actionable preview and rate-limit recovery hints.
Spec source: active@ddbecbafcf0d68cc115faf3fcf0214fb6edc30b284147d33e5c9934e7bad82b4
Emit structured validation, API, network, file, and internal error envelopes for Doc shortcuts so users and agents can recover from failed document workflows using stable type, subtype, param, and code fields.
Add Doc domain errscontract and golangci guards to prevent legacy envelope and common helper regressions.