feat(perf): daily image-prune script, first run reclaimed 28.36GB [#759]

Unused images only (prune -a = not referenced by any container); volumes
and containers NEVER touched. Wired into the daily 09:00 automation.

Details: https://projects.knownelement.com/issues/759#note-1

perf/image-prune.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# perf/image-prune.sh — reclaim docker image slop on the Cloudron host.
+# Removes UNUSED images only (not referenced by ANY container, running or
+# stopped). NEVER touches volumes (app data!) or containers. Cloudron
+# re-pulls an image if an app ever needs it again (costs a pull, no data
+# loss). Founder-directed daily task 2026-09-03 [#758]. First run
+# reclaimed 28.36GB.
+#
+# Run (from a checkout):
+#   VM_IP=my.knownelement.com VM_USER=root timeout 300 bash tests/remote.sh vm-file perf/image-prune.sh
+
+set -u
+
+echo "== before =="
+docker system df | grep -E "Images|Local Volumes"
+df -h / | awk 'NR==2 { printf "root fs: %s used of %s (%s used)\n", $3, $2, $5 }'
+
+echo "== prune (unused images only; volumes/containers NEVER) =="
+docker image prune -af
+
+echo "== after =="
+docker system df | grep -E "Images|Local Volumes"
+df -h / | awk 'NR==2 { printf "root fs: %s used of %s (%s used)\n", $3, $2, $5 }'