feat!: deprecate cookiecutter

[lsorber]

Changes:

• Deprecate Cookiecutter
• Upgrade GitHub Actions
• Upgrade Debian to trixie
• Install Starship with apt instead of as a Dev Containers feature

[Copilot]

Pull request overview

This PR completes the migration from Cookiecutter to Copier by removing all Cookiecutter-related files and documentation. The project now exclusively uses Copier for project scaffolding.

Key Changes:

• Removed Cookiecutter configuration files and post-generation hooks
• Removed migration documentation from README
• Removed Cookiecutter-based test workflow and renamed Copier test workflow to simply "test"
• Updated GitHub Actions versions (checkout@v4 → v6, setup-uv@v6 → v7)

Reviewed changes

Copilot reviewed 32 out of 33 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
hooks/post_gen_project.py	Deleted Cookiecutter post-generation hook script that conditionally removed files based on template configuration
cookiecutter.json	Deleted Cookiecutter template configuration file containing project scaffolding options
copier.yml	Removed trailing empty line (whitespace cleanup)
README.md	Removed "Migrating from Cookiecutter" documentation section and renumbered development environment options
.github/workflows/test.yml	Removed Cookiecutter test job, renamed "test-copier" to "test", and updated GitHub Actions versions
.github/workflows/pr.yml	Updated setup-uv action from v6 to v7

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