Move workspace artifacts to workspace/ subdirectories

Relocate project-specific working files from flat .devagent/ directories into
.devagent/workspace/ subdirectories:

- product/ → workspace/product/ (mission.md, roadmap.md, guiding-questions.md)
- memory/ → workspace/memory/ (constitution.md, decision-journal.md, tech-stack.md, overview.md)
- features/README.md → workspace/features/README.md
- research/ → workspace/research/ (research markdown files)

This separates project-specific artifacts from the portable agent kit, enabling
clean cross-project DevAgent adoption by copying only the core/ directory.

Part of core/workspace split to establish clear 'copy vs customize' boundaries.

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

Author: codegen-sh[bot]