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

docs/explanation/explanation.md

@@ -1,25 +0,0 @@
----
-title: Explanation
-modified: 2026-02-10
-last-reviewed: 2026-02-10
-tags:
-  - explanation
----
-
-# Explanation
-
-Understanding-oriented content explaining the "why" behind BlumeOps design decisions.
-
-## Philosophy
-
-| Article | Description |
-|---------|-------------|
-| [[why-gitops]] | Why infrastructure-as-code and GitOps for a homelab |
-
-## Design
-
-| Article | Description |
-|---------|-------------|
-| [[architecture]] | How all the pieces fit together |
-| [[federated-login]] | How SSO works across BlumeOps (Authentik) |
-| [[security-model]] | Network security, secrets, and access control |