Classify external issues as unconfirmed bugs #11505

Automatically classify issues opened by external contributors as type Bug with the unconfirmed label. Issues opened by owners, members, or collaborators remain unchanged.

Update the bug-report template to explicitly use type Bug and post a short comment explaining that Feature, Task, and Milestone issues are maintainer-managed and feature ideas belong on Mozilla Connect.

Acceptance criteria

• External issues become type Bug and receive unconfirmed.
• Maintainer and collaborator issues remain unchanged.
• Workflow permissions are minimal and action references are SHA-pinned.
• Workflow validation passes.