feat: initial project setup with implementation plan

- Add PLAN.md with 40-step checklist across 10 phases
- Add CLAUDE.md with project-specific instructions
- Set up nix flake with FastHTML/MonsterUI dependencies
- Create Python package skeleton (src/animaltrack)
- Vendor FastHTML and MonsterUI documentation
- Add Docker build configuration

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/mistletoe.nix

@@ -0,0 +1,30 @@
+{
+  lib,
+  buildPythonPackage,
+  fetchPypi,
+  setuptools,
+}:
+
+buildPythonPackage rec {
+  pname = "mistletoe";
+  version = "1.5.0";
+  pyproject = true;
+
+  src = fetchPypi {
+    inherit pname version;
+    hash = "sha256:0i2d4v9rrk6275pjqf816izhv79pbinifdg0hb45w1zqigk3pdv8";
+  };
+
+  build-system = [ setuptools ];
+
+  # No runtime dependencies
+  dependencies = [ ];
+
+  doCheck = false;
+
+  meta = {
+    description = "A fast, extensible Markdown parser in pure Python";
+    homepage = "https://github.com/miyuchina/mistletoe";
+    license = lib.licenses.mit;
+  };
+}