Wire Astro content collections to the shared content/ directory

Adds a custom Astro loader that reads the Zola-style TOML frontmatter
(+++ ... +++) directly, so content/ stays the single source of truth
with no YAML conversion step. Defines writings/portfolio/bookshelf/
blogs collections and basic list/detail pages for each, rendering the
seed entries end-to-end (verified in-browser via `pnpm --filter site
dev`). This is the first runnable MVP: home, writings list + post
detail, portfolio, bookshelf (books and papers), and curated blogs
all render from real content/ files.

apps/site/package.json

@@ -12,7 +12,9 @@
     "astro": "astro"
   },
   "dependencies": {
-    "astro": "^7.2.9"
+    "astro": "^7.2.9",
+    "marked": "^18.0.11",
+    "smol-toml": "^1.8.0"
   },
   "allowScripts": {
     "esbuild": true