docs: add Copilot instructions

[EndlessTrax]

This pull request adds comprehensive documentation for the Python Template Snippets VS Code extension in the .github/copilot-instructions.md file. The new documentation explains the project structure, snippet conventions, available commands, release process, and commit guidelines, providing clear instructions for maintainers and contributors.

Documentation improvements:

• Added a detailed overview of the project structure, including how snippet catalogs are registered and used in package.json.
• Documented conventions for organizing and naming snippet entries, including prefix namespaces and placeholder usage.
• Provided instructions for validating snippet catalogs, building the VSIX package, and the release workflow.
• Clarified release responsibilities and the requirement for matching tag and package.json versions.
• Established strict guidelines for commit and pull request titles using Conventional Commits.

Summary

• add repository-specific Copilot instructions for snippet changes, validation, releases, and contribution titles
• document the strict Conventional Commits requirement for commit and pull request titles

[Copilot]

Pull request overview

Adds repository-specific GitHub Copilot instructions tailored to this project’s reality as a declarative VS Code snippets extension (not a Python runtime), including snippet-catalog conventions, local validation/packaging commands, release safeguards, and Conventional Commits requirements.

Changes:

• Add a new .github/copilot-instructions.md detailing project structure and snippet authoring conventions.
• Document practical validation commands (JSON parse + vsce packaging) that reflect the repo’s lack of npm scripts/tests.
• Codify release/tag safeguards and Conventional Commits requirements for commits and PR titles.

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