docs: update python instructions and add pytest guide

[yxbh]

Summary

Updates agent instructions after evaluating github/awesome-copilot for applicable content.

Changes

python.instructions.md — Updated to match actual repo conventions:

• Line length: 79 → 100 (matches ruff config in pyproject.toml)
• Type hints: typing.List/typing.Dict → modern list[str]/dict[str, int]/X | None
• Added bdpl-specific conventions: from __future__ import annotations, slots=True dataclasses, struct-based binary parsing, typer/rich
• Added ruff check/format as pre-commit step
• Removed over-commenting guidance ("comment every function") in favor of "only comment what needs clarification"

pytest.instructions.md — New file with project-specific testing patterns:

• Documents the fixture architecture (session-scoped disc analysis, _fixture_path/_analyze_fixture helpers)
• Per-disc integration test patterns with examples
• Matrix test conventions (all 6 parametrizations to update)
• Fixture directory structure rules (files go under disc{N}/, NOT disc{N}/BDMV/)
• Running tests cheat sheet

awesome-copilot evaluation

Reviewed ~60 instructions and ~80 skills from github/awesome-copilot. Findings:

Content	Verdict
python.instructions.md	Already had — but ours was the generic version with wrong line length. Now customized.
code-review-generic.instructions.md	Already have — identical to upstream
security-and-owasp.instructions.md	Already have — comprehensive
agent-safety.instructions.md	Not relevant — targets AI agent frameworks (LangChain, etc.)
update-docs-on-code-change	Interesting concept but over-engineered for this project
Framework-specific (React, Angular, ASP.NET, etc.)	Wrong tech stack
No pytest, ruff, typer, or binary-parsing instructions exist	Created our own pytest one