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/error.rs

@@ -0,0 +1,42 @@
+//! Custom error types for the nrd.sh compiler.
+
+use std::path::PathBuf;
+
+/// All errors that can occur during site compilation.
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    /// Failed to read a content file.
+    #[error("failed to read {path}: {source}")]
+    ReadFile {
+        path: PathBuf,
+        #[source]
+        source: std::io::Error,
+    },
+
+    /// Failed to parse frontmatter.
+    #[error("invalid frontmatter in {path}: {message}")]
+    Frontmatter { path: PathBuf, message: String },
+
+    /// Failed to write output file.
+    #[error("failed to write {path}: {source}")]
+    WriteFile {
+        path: PathBuf,
+        #[source]
+        source: std::io::Error,
+    },
+
+    /// Failed to create output directory.
+    #[error("failed to create directory {path}: {source}")]
+    CreateDir {
+        path: PathBuf,
+        #[source]
+        source: std::io::Error,
+    },
+
+    /// Content directory not found.
+    #[error("content directory not found: {0}")]
+    ContentDirNotFound(PathBuf),
+}
+
+/// Result type alias for compiler operations.
+pub type Result<T> = std::result::Result<T, Error>;