fix(powershell): ensure UTF-8 templates are written without BOM (#2280)

* fix(powershell): strip BOM from templates and ensure No-BOM output

* fix: address review feedback on encoding and naming for all ps scripts

* fix: address copilot feedback (encoding detection and variable naming)

* fix: remove duplicate comments in setup-plan.ps1

* test: verify spec.md is written without UTF-8 BOM

* test: also verify BOM-free output under Windows PowerShell 5.1

* fix

* fix: resolve merge conflict with main, add TestDescriptionQuoting

* fix: resolve TestDescriptionQuoting string quoting conflict with main

* test: restore PowerShell prefix-stripping parity test in TestGetFeaturePathsSinglePrefix

* fix: remove trailing whitespace from module docstring blank lines

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* test: seed ps_git_repo with BOM-prefixed template to exercise WriteAllText fix

* fix: remove duplicate ps_git_repo fixture, restore ext_ps_git_repo

* fix: remove unrelated TestDescriptionQuoting and restore original test_ps_specify_feature_prefixed_resolves_by_prefix

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Co-authored-by: Nimraakram22 <nimra.akram123451@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>