Fix frontmatter field name: date-modified -> modified

Quartz's CreatedModifiedDate plugin recognizes `modified`, `lastmod`,
`updated`, and `last-modified` — but not `date-modified`. The wrong
field name caused Quartz to fall through to filesystem timestamps
(UTC in Dagger), showing incorrect dates on the rendered site.

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

docs/reference/infrastructure/gandi.md

@@ -1,6 +1,6 @@
 ---
 title: Gandi
-date-modified: 2026-02-08
+modified: 2026-02-08
 tags:
   - infrastructure
   - networking

docs/reference/infrastructure/gilbert.md

@@ -1,6 +1,6 @@
 ---
 title: Gilbert
-date-modified: 2026-02-07
+modified: 2026-02-07
 tags:
   - infrastructure
   - host

docs/reference/infrastructure/hosts.md

@@ -1,6 +1,6 @@
 ---
 title: Hosts
-date-modified: 2026-02-10
+modified: 2026-02-10
 tags:
   - infrastructure
 ---

docs/reference/infrastructure/indri.md

@@ -1,6 +1,6 @@
 ---
 title: Indri
-date-modified: 2026-02-09
+modified: 2026-02-09
 tags:
   - infrastructure
   - host

docs/reference/infrastructure/power.md

@@ -1,6 +1,6 @@
 ---
 title: Power
-date-modified: 2026-02-09
+modified: 2026-02-09
 tags:
   - infrastructure
 ---

docs/reference/infrastructure/routing.md

@@ -1,6 +1,6 @@
 ---
 title: Routing
-date-modified: 2026-02-09
+modified: 2026-02-09
 tags:
   - infrastructure
   - networking

docs/reference/infrastructure/tailscale.md

@@ -1,6 +1,6 @@
 ---
 title: Tailscale
-date-modified: 2026-02-08
+modified: 2026-02-08
 tags:
   - infrastructure
   - networking

docs/reference/infrastructure/unifi.md

@@ -1,6 +1,6 @@
 ---
 title: UniFi
-date-modified: 2026-02-10
+modified: 2026-02-10
 tags:
   - infrastructure
   - networking