Astro garden skeleton: Quartz-compatible URLs, Obsidian markdown, odometer homepage

- glob content loader over the vault's Digital_Garden with verbatim-path ids
- remark plugin: wikilinks, image embeds (wiki + relative md), callouts
- URL parity with live Quartz sitemap verified 102/102 (built set is a
  strict superset: +30 synthetic folder listings)
- KaTeX, lazy mermaid, shiki dual themes, quoted-string draft handling
- copy-assets resolves embeds vault-wide (fixes live-site 404s for
  Blog Scratch/assets images) into flat /assets/
- homepage odometer widget: fetches api.c0smere.net baseline, ticks at
  lifetime average rate, fail-closed hidden on error
- deploy/: nginx try_files config + compose (port 18100) + containerized
  build script for cyrion

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

astro.config.mjs

@@ -0,0 +1,20 @@
+import { defineConfig } from 'astro/config';
+import remarkMath from 'remark-math';
+import rehypeKatex from 'rehype-katex';
+import { remarkObsidian } from './src/lib/remark-obsidian.mjs';
+import { buildSlugMap, CONTENT_DIR } from './src/lib/quartz-compat.mjs';
+
+const slugMap = buildSlugMap(CONTENT_DIR);
+
+export default defineConfig({
+  site: 'https://garden.c0smere.net',
+  // 'file' => /A/B.html served as /A/B — matches Quartz's URL shape
+  build: { format: 'file' },
+  markdown: {
+    remarkPlugins: [[remarkObsidian, { slugMap }], remarkMath],
+    rehypePlugins: [rehypeKatex],
+    shikiConfig: {
+      themes: { light: 'github-light', dark: 'github-dark' },
+    },
+  },
+});