Remove obsolete PowerShell scripts and templates for feature management

- Deleted `create-new-feature.ps1`, `setup-plan.ps1`, and `update-agent-context.ps1` scripts as they are no longer needed.
- Removed associated templates: `agent-file-template.md`, `constitution-template.md`, `plan-template.md`, `spec-template.md`, and `tasks-template.md`.
- This cleanup helps streamline the project by eliminating unused code and templates.

Author: MariusStorhaug