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/templates.rs

@@ -0,0 +1,63 @@
+//! HTML templates using maud.
+
+use crate::content::Frontmatter;
+use maud::{html, Markup, DOCTYPE};
+
+/// Render a blog post with the base layout.
+pub fn render_post(frontmatter: &Frontmatter, content_html: &str) -> Markup {
+    base_layout(
+        &frontmatter.title,
+        html! {
+            article.post {
+                header {
+                    h1 { (frontmatter.title) }
+                    @if let Some(ref date) = frontmatter.date {
+                        time.date { (date) }
+                    }
+                    @if let Some(ref desc) = frontmatter.description {
+                        p.description { (desc) }
+                    }
+                    @if !frontmatter.tags.is_empty() {
+                        ul.tags {
+                            @for tag in &frontmatter.tags {
+                                li { a href=(format!("/tags/{}/", tag)) { (tag) } }
+                            }
+                        }
+                    }
+                }
+                section.content {
+                    (maud::PreEscaped(content_html))
+                }
+            }
+        },
+    )
+}
+
+/// Base HTML layout wrapper.
+fn base_layout(title: &str, content: Markup) -> Markup {
+    html! {
+        (DOCTYPE)
+        html lang="en" {
+            head {
+                meta charset="utf-8";
+                meta name="viewport" content="width=device-width, initial-scale=1";
+                title { (title) " | nrd.sh" }
+                link rel="stylesheet" href="/style.css";
+            }
+            body {
+                nav {
+                    a href="/" { "nrd.sh" }
+                    a href="/blog/" { "blog" }
+                    a href="/projects/" { "projects" }
+                    a href="/about/" { "about" }
+                }
+                main {
+                    (content)
+                }
+                footer {
+                    p { "© nrdxp" }
+                }
+            }
+        }
+    }
+}