Set up Copilot instructions

[Copilot]

Description

Adds .github/copilot-instructions.md to provide repository-specific context for GitHub Copilot, as documented in Best practices for Copilot coding agent.

The instructions cover:

• Project overview (MMM, CLV, Customer Choice, Bass models)
• Project structure and key directories
• Development setup (conda, make init, pre-commit)
• Code style (Ruff, mypy, NumPy docstrings with both doctest and Sphinx code block examples, type hints)
• Testing and documentation commands (with environment activation guidance)
• Common patterns for model building and priors (using pymc_extras.prior)
• Key dependencies (referencing pyproject.toml for versions)
• PR guidelines and resource links

Related Issue

• Closes [pre-commit.ci] pre-commit autoupdate #2102

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a relevant logical change

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #2105

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[williambdean]

@copilot There are some incoming PR that use CLAUD . I want to make sure that there is no duplicated work.