docs: add contributing guide

[zichen0116]

Summary

• Add a CONTRIBUTING.md with development setup instructions, code style guidelines, check commands, and PR conventions.
• Derived from the existing .gemini/styleguide.md, pyproject.toml, and CI workflow configuration.

Motivation

The repository currently lacks a contributing guide. New contributors have to inspect the CI configuration and style guide to understand the expected tooling, code style, and PR format. A dedicated CONTRIBUTING.md lowers the barrier to entry and consolidates this information in one place.

What the guide covers

• Prerequisites: Python 3.10+, uv, CUDA-capable GPU
• Development setup: Fork, clone, uv sync --all-extras --dev
• Code style: Ruff formatting, Ruff linting with import sorting, ty type checking, naming/comment/annotation conventions from the style guide
• Running checks: All four CI commands (ruff format --check, ruff check, ty check, uv build)
• PR guidelines: One change per PR, semantic PR title prefixes (feat:, fix:, perf:, docs:, build:), no unrelated code modifications
• Issue reporting: Search existing issues first, include environment details
• License: AGPL-3.0-or-later contribution agreement

Verification

• The guide references commands and conventions that match the actual CI workflow (.github/workflows/ci.yml), style guide (.gemini/styleguide.md), and project configuration (pyproject.toml).
• No existing open PR adds a contributing guide.
• Only Markdown files are changed; no code modifications.

[gemini-code-assist]

Code Review

This pull request introduces a new CONTRIBUTING.md file that outlines the contribution guidelines, development workflow, and code style requirements for the Heretic project. The review feedback correctly identifies an incorrect URL for the ty type-checking tool and provides a suggestion to point to the correct repository.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[gemini-code-assist]

The URL for ty is incorrect. ty is maintained by p-e-w, not astral-sh. It should point to https://github.com/p-e-w/ty.

Suggested change

	- **Type checking**: All code is type-checked with [ty](https://github.com/astral-sh/ty). Run `uv run ty check .` to verify.
	- **Type checking**: All code is type-checked with [ty](https://github.com/p-e-w/ty). Run `uv run ty check .` to verify.