Backfill date-modified frontmatter on all docs

Dagger's --src=. excludes .git, so Quartz can't use git history for
page dates inside containers. Populate date-modified: YYYY-MM-DD in
frontmatter for all 80 doc articles so the frontmatter priority level
(highest in quartz.config.ts) works with or without git.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/tutorials/replication/argocd-config.md

@@ -1,5 +1,6 @@
 ---
 title: ArgoCD Config
+date-modified: 2026-02-07
 tags:
   - tutorials
   - replication

docs/tutorials/replication/core-services.md

@@ -1,5 +1,6 @@
 ---
 title: Core Services
+date-modified: 2026-02-07
 tags:
   - tutorials
   - replication

docs/tutorials/replication/kubernetes-bootstrap.md

@@ -1,5 +1,6 @@
 ---
 title: Kubernetes Bootstrap
+date-modified: 2026-02-07
 tags:
   - tutorials
   - replication

docs/tutorials/replication/observability-stack.md

@@ -1,5 +1,6 @@
 ---
 title: Observability Stack
+date-modified: 2026-02-07
 tags:
   - tutorials
   - replication

docs/tutorials/replication/tailscale-setup.md

@@ -1,5 +1,6 @@
 ---
 title: Tailscale Setup
+date-modified: 2026-02-07
 tags:
   - tutorials
   - replication