#963 Allow changing an existing issue to security/confidential after submission

Summary

Once an issue has been filed, there should be a way for the author or an admin to mark it as security-sensitive or confidential after the fact.

Current behavior

swamp issue edit <number> only supports changing the title and body. If a user files an issue and later realizes it contains security-sensitive or confidential information, there is no documented way to change the issue type or visibility.

Expected behavior

Provide a supported workflow to mark an existing issue as security, confidential, or equivalent after submission. This should restrict visibility appropriately and avoid requiring users to create a second issue manually.

Why this matters

Users can misclassify an issue initially, or discover later that the issue includes sensitive reproduction details. They need a safe remediation path that does not leave sensitive information exposed.

Suggested approach

Add a CLI/API path such as swamp issue edit <number> --type security, swamp issue edit <number> --confidential, or a dedicated command for visibility/type changes, with suitable authorization checks.