Convert to setuptools-scm for automatic version management

- Remove manual version management in favor of git tag-based versioning
- Simplify __version__.py to import from generated _version.py
- Add simplified release_simplified.py script
- Add _version.py to .gitignore (auto-generated)

🚀 Generated with [Claude Code](https://claude.ai/code)

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

pyproject.toml

@@ -1,10 +1,10 @@
 [build-system]
-requires = ["setuptools>=61.0"]
+requires = ["setuptools>=64", "setuptools-scm>=8"]
 build-backend = "setuptools.build_meta"
 
 [project]
 name = "markitect"
-version = "0.7.0"
+dynamic = ["version"]
 description = "Advanced Markdown engine for structured content"
 readme = "README.md"
 requires-python = ">=3.8"
@@ -100,3 +100,6 @@ module = [
     "yaml.*"
 ]
 ignore_missing_imports = true
+
+[tool.setuptools_scm]
+write_to = "markitect/_version.py"