Implement unassign_issue Tool to Remove Specific Contributors

Currently, we can add assignees or clear all assignees from an issue. However, we lack the functionality to remove one or more specific contributors while leaving others assigned. This is a necessary feature for complete assignee management.

Proposed Solution

Create a new tool named unassign_issue that removes a specified list of assignees from an issue.

1. Update OpenAPI Spec:

• In ai/mcp/server/github-workflow/openapi.yaml, add a delete operation to the /issues/{issue_number}/assignees path.
• The operationId will be unassign_issue.
• The request body will take an assignees array (the logins to remove).

2. Update IssueService.mjs:

• Create a new unassignIssue method.
• This method must perform the same permission check as the assignIssue tool, verifying the user has ADMIN, MAINTAIN, or WRITE permissions by checking RepositoryService.viewerPermission.
• If the check passes, the method will use the gh issue edit <number> --remove-assignee <login> command to remove the specified users.

3. Update toolService.mjs:

• Add unassign_issue to the serviceMapping to expose the new method as a tool.