fix: strip HTML tags from derived excerpt

gsub(/<[^>]+>/, '') before stripping markdown markup, so auto-generated
excerpts don't leak raw HTML from post bodies.

test/post_test.rb

@@ -30,6 +30,12 @@ class PostTest < Minitest::Test
     assert_equal "The body paragraph.", post.excerpt
   end
 
+  def test_derived_excerpt_strips_html_tags
+    body = "<strong>Bold</strong> and <em>italic</em>."
+    post = Volumen::Post.new(metadata: { "slug" => "x" }, body: body)
+    assert_equal "Bold and italic.", post.excerpt
+  end
+
   def test_summary_shape
     post = Volumen::Post.new(metadata: { "slug" => "x", "title" => "X" }, body: "Body")
     summary = post.summary