Fix spider trap: disable SPA mode, remove index files, relax wiki-links (#290)

## Summary

Fixes the Facebook crawler spider trap that's been generating infinite recursive URLs like `/how-to/tutorials/tutorials/how-to/explanation/...` for several days.

**Root cause:** Quartz SPA mode + nginx `try_files` fallback to `index.html` meant any fabricated URL returned the root HTML shell with HTTP 200. Crawlers followed relative links from those fake URLs, creating infinite recursion.

**Fix:**
- Disable Quartz SPA mode (`enableSPA: false`) — all pages are now fully static HTML
- Replace nginx SPA fallback with `=404` + Quartz's static `404.html`
- Remove `robots.txt` exclusions (no longer needed)

**Docs cleanup (Obsidian.nvim compat no longer needed):**
- Delete hand-curated category index files (`tutorials.md`, `reference.md`, `how-to.md`, `explanation.md`) — Quartz auto-generates folder pages
- Delete `postgresql-storage.md` (redirect stub) and `migrate-forgejo-from-brew.md` (stale history)
- Drop `docs-check-index` and `docs-check-filenames` prek hooks
- Rewrite `docs-check-links` to allow path-based wiki-links (`[[path/to/file]]`) and only error on true ambiguity
- Add `ai-docs` doc tree listing to replace index files for AI context
- Add natural cross-links from reference cards to fix orphan docs

## Deployment and Testing

- [ ] Merge and let the build pipeline run
- [ ] Verify docs.eblu.me serves pages correctly with full page loads
- [ ] Verify non-existent URLs return 404
- [ ] Monitor crawler traffic — should drop to near zero for fabricated URLs

Reviewed-on: #290

containers/quartz/default.conf

@@ -14,18 +14,16 @@ server {
         add_header Cache-Control "public, immutable";
     }
 
-    # Serve robots.txt inline to prevent crawlers from entering /explore/ and /tags/,
-    # which is an SPA feature that generates infinite relative-link trees
-    # when crawled (the March 2026 spider-trap incident).
-    location = /robots.txt {
-        default_type text/plain;
-        return 200 "User-agent: *\nDisallow: /explore/\nDisallow: /tags/\n";
+    # Static file serving — no SPA fallback.
+    # Quartz generates complete HTML for every page, so all valid URLs
+    # map to real files. Non-existent paths get 404.html (generated by
+    # Quartz's NotFoundPage plugin), preventing the spider-trap issue
+    # where crawlers would get index.html for fabricated URLs.
+    location / {
+        try_files $uri $uri/ $uri.html =404;
     }
 
-    # SPA fallback - serve index.html for client-side routing
-    location / {
-        try_files $uri $uri/ $uri.html /index.html;
-    }
+    error_page 404 /404.html;
 
     # Health check endpoint
     location /healthz {