[codex] add CI and core tests

[offbyonebit]

What changed

• Added a GitHub Actions CI workflow for Python 3.10, 3.11, and 3.12.
• Added pytest coverage for config persistence/model lookup, HF spec parsing, GGUF discovery/registration, router residency policy, and OpenAI proxy behavior.
• Added mocked streaming proxy coverage for stream: true SSE forwarding and upstream cleanup.
• Added pytest configuration and the build dev dependency so CI can verify package artifacts.
• Cleaned up existing Ruff violations and removed redundant Hatch static-file include config that caused duplicate wheel entries.
• Ignored local tool caches such as .uv-cache, .ruff_cache, and .mypy_cache.

Why

This gives the alpha core a software-side safety net before hardware-specific validation. The suite avoids Arc hardware, real GGUF downloads, and real llama-server subprocesses by using temporary files and fakes around process/network boundaries.

Validation

• uv --cache-dir .uv-cache run --python 3.12 --with-editable '.[dev]' ruff check .
• uv --cache-dir .uv-cache run --python 3.12 --with-editable '.[dev]' pytest
• uv --cache-dir .uv-cache run --python 3.12 --with-editable '.[dev]' python -m build
• Confirmed the built wheel still contains arc_llama/static/app.js, index.html, and style.css.