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/launchd/se.jordbo.mempalace-pi-session.plist

@@ -0,0 +1,77 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
+  "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <!--
+      se.jordbo.mempalace-pi-session.plist — macOS launchd user agent
+      that mines pi coding-agent session history into MemPalace weekly.
+
+      Template: replace USER with your macOS short username before installing.
+      The install recipe in contrib/README.md does this for you via `sed`.
+
+      Parity with contrib/systemd/mempalace-pi-session.{service,timer}:
+        - Weekly Tue 03:00 local time → StartCalendarInterval below.
+          (Staggered from the opencode one, which runs Mon 03:00.)
+        - Low-priority background I/O → ProcessType=Background + LowPriorityIO.
+        - Single-instance guard → launchd refuses to start a second copy of
+          the same Label while one is running.
+        - "Skip if pi has never been used" → no native equivalent.
+          mempalace-pi-session exits cleanly (zero drawers filed, fast) when
+          ~/.pi/agent/sessions is absent, so no guard is strictly needed.
+    -->
+
+    <key>Label</key>
+    <string>se.jordbo.mempalace-pi-session</string>
+
+    <key>ProgramArguments</key>
+    <array>
+        <string>/Users/USER/.local/bin/mempalace-pi-session</string>
+    </array>
+
+    <!-- launchd gives agents a minimal PATH. mempalace-pi-session invokes
+         `mempalace` and `python3`, both must resolve. -->
+    <key>EnvironmentVariables</key>
+    <dict>
+        <key>PATH</key>
+        <string>/Users/USER/.local/bin:/opt/homebrew/bin:/usr/local/bin:/usr/bin:/bin</string>
+        <key>HOME</key>
+        <string>/Users/USER</string>
+    </dict>
+
+    <!-- Weekly, Tuesday 03:00 local time. Staggered from the opencode job
+         (Mon 03:00) so a machine with both installed won't race the HNSW
+         repair step. -->
+    <key>StartCalendarInterval</key>
+    <dict>
+        <key>Weekday</key>
+        <integer>2</integer>
+        <key>Hour</key>
+        <integer>3</integer>
+        <key>Minute</key>
+        <integer>0</integer>
+    </dict>
+
+    <key>RunAtLoad</key>
+    <false/>
+
+    <key>ProcessType</key>
+    <string>Background</string>
+    <key>LowPriorityIO</key>
+    <true/>
+    <key>Nice</key>
+    <integer>10</integer>
+
+    <!-- Runaway guard. Pi corpora are typically smaller than opencode's
+         (short tactical sessions), so 2h is generous. -->
+    <key>ExitTimeOut</key>
+    <integer>7200</integer>
+
+    <!-- Logs. Tail with:
+           tail -f ~/Library/Logs/mempalace-pi-session.log -->
+    <key>StandardOutPath</key>
+    <string>/Users/USER/Library/Logs/mempalace-pi-session.log</string>
+    <key>StandardErrorPath</key>
+    <string>/Users/USER/Library/Logs/mempalace-pi-session.err.log</string>
+</dict>
+</plist>