fix: return project item id usable for updates

[njoppi2]

Summary

• include fullDatabaseId in the GraphQL response for projects_write / add_project_item
• return a numeric item_id that can be passed directly to update_project_item and delete_project_item
• keep the existing node id in the response for backwards compatibility

Closes #2153.

Validation

• ran go test ./pkg/github -run "Test_ProjectsWrite_AddProjectItem|Test_ProjectsWrite_UpdateProjectItem" in a Go 1.24 Docker container
• formatted touched files with gofmt in the same container

[SamMorrowDrums]

Clean fix for #2153 — adds fullDatabaseId to the mutation and returns both full_database_id (string) and item_id (int64) so the result can be passed straight into update_project_item / delete_project_item. Existing node id preserved for backwards compat. Rebased onto main, tests pass, lint clean. Thanks!