Set up e2e testing

tests/data/sites/main/config.yml

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+name: Main Test Site
+base_url: http://127.0.0.1:8000
+template: plain

tests/data/sites/main/content/first_post.md

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+---
+type: article
+section: posts
+slug: first-post
+title: First Post
+date: 2026-08-28
+tags: [meta]
+---
+Welcome to the test site. This site will contain some test content for the
+static site generator, the content won't matter a lot.

tests/e2e/test_success.py

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+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+MAIN_SITE = Path(__file__).parent.parent / "data" / "sites" / "main"
+
+
+@pytest.fixture(scope="module")
+def built_site(tmp_path_factory):
+    site_dir = tmp_path_factory.mktemp("main")
+    shutil.copytree(
+        MAIN_SITE,
+        site_dir,
+        dirs_exist_ok=True,
+        ignore=shutil.ignore_patterns("output", "build.log"),
+    )
+    result = subprocess.run(
+        ["ossg", "--site", str(site_dir), "build"],
+        capture_output=True,
+        text=True,
+    )
+    assert result.returncode == 0, result.stderr
+    return site_dir / "output"
+
+
+def test_article_renders(built_site):
+    output = built_site / "posts" / "first-post" / "index.html"
+    assert output.exists()
+    assert "First Post" in output.read_text()