disable sharing by default

[zach-fuller]

Summary

This PR changes the default sharing behavior from manual to disabled. Given the sensitive nature of most conversations, requiring explicit configuration to enable sharing is a more sensible default.

This change builds trust and improves security while still allowing users who have read the documentation to enable the feature if they wish.

Changes

• Adds default logic to set share: "disabled" when no explicit configuration is provided.
• Updates documentation to reflect that "disabled" is now the default sharing mode.