fix: include migration scripts in release packages; use release tag for version stamp

- Copy migrate-to-documentation.ps1/.sh into scripts/powershell/ and scripts/bash/
  so they are bundled in release zip packages (previously they lived only in
  .documentation/scripts/ which is not included in the build)
- run_migration_script() now checks .documentation/scripts/{powershell,bash}/ first
  (new install location) then falls back to .documentation/scripts/ (legacy)
- download_and_extract_template() now returns (project_path, release_tag)
- write_version_stamp() accepts release_version param and prefers it over CLI
  metadata version — stamp now always reflects the downloaded template release

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: markhazleton