Fix frontmatter field name for Quartz date display (#158)

## Summary

- Rename `date-modified` -> `modified` in all 80 docs and the `docs-check-frontmatter` task

Quartz's `CreatedModifiedDate` plugin recognizes `modified`, `lastmod`, `updated`, and `last-modified` — but not `date-modified`. The wrong field name caused Quartz to ignore frontmatter dates entirely and fall through to filesystem timestamps (UTC inside Dagger), showing Feb 12 on pages built late on Feb 11 PST.

## Test plan

- [x] `mise run docs-check-frontmatter` passes
- [ ] Kick off docs release after merge — verify rendered dates match frontmatter values

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/158

docs/tutorials/adding-a-service.md

@@ -1,6 +1,6 @@
 ---
 title: Adding a Service
-date-modified: 2026-02-07
+modified: 2026-02-07
 tags:
   - tutorials
   - argocd

docs/tutorials/ai-assistance-guide.md

@@ -1,6 +1,6 @@
 ---
 title: AI Assistance Guide
-date-modified: 2026-02-09
+modified: 2026-02-09
 tags:
   - tutorials
   - ai

docs/tutorials/contributing.md

@@ -1,6 +1,6 @@
 ---
 title: Contributing
-date-modified: 2026-02-07
+modified: 2026-02-07
 tags:
   - tutorials
   - contributing

docs/tutorials/exploring-the-docs.md

@@ -1,6 +1,6 @@
 ---
 title: Exploring the Docs
-date-modified: 2026-02-10
+modified: 2026-02-10
 tags:
   - tutorials
   - getting-started

docs/tutorials/replicating-blumeops.md

@@ -1,6 +1,6 @@
 ---
 title: Replicating BlumeOps
-date-modified: 2026-02-07
+modified: 2026-02-07
 tags:
   - tutorials
   - replication

docs/tutorials/replication/argocd-config.md

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

docs/tutorials/replication/core-services.md

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

docs/tutorials/replication/kubernetes-bootstrap.md

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

docs/tutorials/replication/observability-stack.md

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

docs/tutorials/replication/tailscale-setup.md

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

docs/tutorials/tutorials.md

@@ -1,6 +1,6 @@
 ---
 title: Tutorials
-date-modified: 2026-02-07
+modified: 2026-02-07
 tags:
   - tutorials
 ---