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.

Cargo.toml

@@ -9,3 +9,5 @@ version     = "0.1.0"
 gray_matter    = "0.2"
 maud           = "0.26"
 pulldown-cmark = "0.12"
+thiserror      = "2"
+walkdir        = "2"