chore: Strip notebook outputs and add nbdev config

- Run nbdev_clean to strip notebook outputs (fixes CI)
- Add generated pyproject.toml from nbdev
- Install nbdev git hooks

This fixes the "unstripped notebooks" CI failure.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

nbs/index.ipynb

@@ -532,4 +532,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}

pyproject.toml

@@ -0,0 +1,11 @@
+[build-system]
+requires = ["setuptools>=64.0"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name="sec-downloader"
+requires-python=">=3.7"
+dynamic = [ "keywords", "description", "version", "dependencies", "optional-dependencies", "readme", "license", "authors", "classifiers", "entry-points", "scripts", "urls"]
+
+[tool.uv]
+cache-keys = [{ file = "pyproject.toml" }, { file = "settings.ini" }, { file = "setup.py" }]