Add README with uv install instructions, fix pyproject.toml

- Add README.md with install/run instructions using uv
- Add build-system and script entry point to pyproject.toml
- Add .gitignore for __pycache__ and save files
- Remove placeholder main.py (entry point is h2g2.main:main)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

uv.lock

@@ -0,0 +1,8 @@
+version = 1
+revision = 3
+requires-python = ">=3.12"
+
+[[package]]
+name = "h2g2"
+version = "0.1.0"
+source = { editable = "." }