Add mypy type checking to pre-commit hooks

[Copilot]

PR Checklist

• Tests pass
• Docs if needed
• pre-commit passes

Changes

Adds mypy type checking as a pre-commit hook with minimal configuration enforcing typed function definitions.

Configuration in pyproject.toml:

• python_version = "3.9" - matches minimum supported version
• disallow_untyped_defs = true - enforces type hints on all functions
• warn_return_any = true - warns on Any returns
• warn_unused_configs = true - validates config

Pre-commit hook:

• Uses pre-commit/mirrors-mypy v1.14.1
• No additional dependencies (template has none currently)

Code updates:

• Fixed test_hello() to include -> None return type annotation
• Updated README to reflect mypy is now enforced

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Repo] Should we use types and check with mypi?</issue_title>
<issue_description>We can decide if it makes sense for us to force ourself to use types hints and check them with mypi</issue_description>

<agent_instructions>add mypy to precommit as a new hook and add minimal mypy cofig in pyproject.toml</agent_instructions>

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

• Fixes [Repo] Should we use types and check with mypi? #4

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[Copilot]

Pull request overview

This PR adds mypy static type checking to the project's pre-commit hooks to enforce type hints across the codebase. The implementation includes minimal mypy configuration that requires type annotations on all function definitions.

Key changes:

• Added mypy as a pre-commit hook using v1.14.1
• Configured mypy in pyproject.toml with strict typing requirements (disallow_untyped_defs = true)
• Added missing return type annotation to test function

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
.pre-commit-config.yaml	Added mypy hook configuration for static type checking
pyproject.toml	Added mypy configuration section with Python 3.9 target and strict typing rules
test/test_hello.py	Added -> None return type annotation to test_hello() function
README.md	Updated documentation to reflect mypy enforcement via pre-commit hooks

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[Copilot]

The mypy pre-commit hook may not work correctly with this project's src/ layout without additional configuration. When mypy runs via pre-commit, it needs to know about the package structure. Consider adding either:

1. An args parameter with --namespace-packages and --explicit-package-bases, or
2. An additional_dependencies parameter if the package needs to be installed

For a src/ layout, you typically need to add args: [--namespace-packages, --explicit-package-bases] to ensure mypy can properly resolve imports from the sandbox package.

Suggested change

	- id: mypy
	- id: mypy
	args: [--namespace-packages, --explicit-package-bases]