docs(contrib): scheduling templates for mempalace-pi-session

Drop-in equivalents of the opencode templates for each scheduler
mechanism:

  systemd/mempalace-pi-session.{service,timer}
  launchd/se.jordbo.mempalace-pi-session.plist
  cron/mempalace-pi-session.cron

Schedule is staggered from the opencode jobs (Mon 03:00 -> Tue 03:00)
so machines running both don't race each other on the post-mine HNSW
repair step. Service unit uses ConditionPathExists=%h/.pi/agent/sessions
to no-op silently on machines that haven't used pi, matching the
opencode template's guard on ~/.local/share/opencode/opencode.db.

contrib/README.md grows a 'Templates at a glance' table so the set is
discoverable without reading the whole doc.

contrib/cron/mempalace-pi-session.cron

@@ -0,0 +1,18 @@
+# Sample crontab entry for mempalace-pi-session.
+#
+# Runs a full pi → MemPalace mine weekly (Tuesdays at 03:00 local).
+# Staggered from mempalace-session.cron (Mondays) so if both are installed
+# they don't race the post-mine HNSW repair step.
+#
+# To install:
+#   (crontab -l 2>/dev/null; cat contrib/cron/mempalace-pi-session.cron) | crontab -
+#
+# To remove, edit your crontab:
+#   crontab -e
+#
+# Replace USER with your actual username.
+
+PATH=/home/USER/.local/bin:/usr/local/bin:/usr/bin:/bin
+
+# m h  dom mon dow  command
+0 3 * * 2  mempalace-pi-session >> /home/USER/.cache/mempalace-pi-session/cron.log 2>&1