feat: implement core markdown rendering pipeline

Add minimal e2e pipeline to transform content → HTML:

- src/error.rs: Custom error types with thiserror
- src/content.rs: YAML frontmatter parsing via gray_matter
- src/render.rs: Markdown → HTML via pulldown-cmark
- src/templates.rs: Maud templates for base layout and posts
- src/main.rs: Pipeline orchestrator

All 15 blog posts successfully rendered to public/blog/*/index.html.
Added thiserror and walkdir dependencies.
Added public/ and DepMap fragment to .gitignore and AGENTS.md.

src/render.rs

@@ -0,0 +1,32 @@
+//! Markdown to HTML rendering via pulldown-cmark.
+
+use pulldown_cmark::{html, Options, Parser};
+
+/// Render markdown content to HTML.
+///
+/// Currently performs basic rendering. Future phases will intercept
+/// code blocks for Tree-sitter highlighting.
+pub fn markdown_to_html(markdown: &str) -> String {
+    let options = Options::ENABLE_TABLES
+        | Options::ENABLE_FOOTNOTES
+        | Options::ENABLE_STRIKETHROUGH
+        | Options::ENABLE_TASKLISTS;
+
+    let parser = Parser::new_ext(markdown, options);
+    let mut html_output = String::new();
+    html::push_html(&mut html_output, parser);
+    html_output
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_basic_markdown() {
+        let md = "# Hello\n\nThis is a *test*.";
+        let html = markdown_to_html(md);
+        assert!(html.contains("<h1>Hello</h1>"));
+        assert!(html.contains("<em>test</em>"));
+    }
+}