Move feature hub template to core/templates/feature-hub-template/

Relocate the feature hub template from .devagent/features/_template/ to
.devagent/core/templates/feature-hub-template/ to position it as a reusable
scaffold in the portable agent kit.

The feature hub template provides the standard structure for organizing feature
development work including research and spec subdirectories.

Part of core/workspace split to enable easy feature hub creation across projects.

Co-authored-by: Jake Ruesink <jaruesink@gmail.com>

Author: codegen-sh[bot]