Add structured error context details to all commands

[AndreyVMarkelov]

Summary

• Add operation, path, revision, email, member_id, from_path/to_path, url, and retry_after_seconds context to JSON error details across all commands (get, put, rm, restore, search, share-link create/list/download/info/revoke/update, cp, mv, team add/remove-member)
• Add detailedError type and withJSONErrorDetails for wrapping errors with context without overriding the error code
• Walk wrapped and joined errors (Unwrap() error + Unwrap() []error) to collect details from all layers
• Add jsonErrorDetailsForCommand with auto-attach of --as-member member_id on Dropbox API errors
• Fix false-positive api_summary detection for local path error messages
• Add golden error output contract tests (testdata/json_contract/error_outputs.json)
• Add subprocess integration tests validating real CLI error JSON against the schema
• Add "JSON contract stability" section to README

Test plan

• go test ./... passes
• golangci-lint run clean
• Golden error output contract audit passes
• Subprocess integration tests pass (unknown command, auth required, structured output unsupported)
• TestJSONErrorDetailsDoesNotTreatLocalPathAsAPISummary confirms false-positive fix
• Team add/remove-member tests verify email details
• All share-link command tests verify url/operation details