feat(monitoring): auto-prune Docker + node_modules, 15-min disk check interval

- check-disk-space.sh: always prune dangling images, unused volumes, and
  build cache >7 days on every run (not just at critical threshold)
- check-disk-space.sh: auto-remove node_modules if found on server
  (never needed — Docker builds inside containers)
- disk-check launchd: reduce interval from 60min to 15min to catch
  disk issues faster (yesterday we hit 100% before hourly check caught it)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/mac-mini/launchd/com.manacore.disk-check.plist

@@ -11,9 +11,9 @@
         <string>/Users/mana/projects/manacore-monorepo/scripts/mac-mini/check-disk-space.sh</string>
     </array>
 
-    <!-- Run hourly -->
+    <!-- Run every 15 minutes — frequent enough to catch disk issues early -->
     <key>StartInterval</key>
-    <integer>3600</integer>
+    <integer>900</integer>
 
     <!-- Also run at startup -->
     <key>RunAtLoad</key>