Add macOS launchd template, bringing automation parity to macOS

Ship a launchd user agent plist alongside the existing systemd and
cron templates so macOS users can schedule mempalace-session without
falling back to cron. launchd is the macOS-native equivalent of a
systemd user timer: same scheduling model, same log conventions, same
single-instance guarantees.

- contrib/launchd/se.jordbo.mempalace-session.plist:
  - Label uses reverse-DNS from the jordbo.se domain for consistency
    with other user-installed launchd jobs; fork the prefix if reusing
    this template in a different org.
  - ProgramArguments points at /Users/USER/.local/bin/mempalace-session
    (USER is substituted at install time, same pattern as
    contrib/cron/).
  - EnvironmentVariables.PATH covers ~/.local/bin, Apple Silicon
    Homebrew, Intel Homebrew, and system defaults — launchd agents
    get a minimal PATH by default and the wrapper needs to find
    mempalace + python3.
  - StartCalendarInterval matches systemd unit's schedule: Monday
    03:00 local.
  - RunAtLoad=false — load shouldn't trigger a run; schedule does.
  - ProcessType=Background + LowPriorityIO=true + Nice=10 mirror
    the systemd unit's Nice=10 + IOSchedulingClass=idle. macOS's
    automatic App Nap and resource throttling for Background jobs
    yields to interactive work cleanly.
  - ExitTimeOut=7200 matches systemd's TimeoutStartSec=7200.
  - StandardOut/ErrorPath under ~/Library/Logs/ so Console.app
    surfaces them.

- contrib/README.md gains a full launchd section:
  - Caveat table comparing to systemd (Persistent=true isn't quite
    matched; RandomizedDelaySec has no equivalent; overlap prevention
    is automatic).
  - Install recipe using launchctl bootstrap (modern) with a fallback
    note for legacy launchctl load -w on older macOS.
  - Verify section shows launchctl list, launchctl print, log tails,
    and launchctl kickstart for manual testing.
  - Uninstall via launchctl bootout.
  - Chooser table updated: macOS now explicitly points at launchd,
    not cron.

- ARCHITECTURE.md §5, SKILL.md Quick automation pitch, and README.md
  Keeping it fresh section all updated to mention the three scheduler
  options and give per-platform quick-starts.

Plist XML validated with plistlib.

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

@@ -0,0 +1,99 @@
+<?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-session.plist — macOS launchd user agent
+      that mines opencode 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-session.{service,timer}:
+        - Weekly Mon 03:00 local time → StartCalendarInterval below.
+        - Low-priority background I/O → ProcessType=Background + LowPriorityIO.
+        - "Fire on next wake if missed" → launchd fires StartCalendarInterval
+          jobs as soon as the system is running past the scheduled time.
+          (Close to systemd's Persistent=true, not identical: if the Mac is
+          fully off at the scheduled time, the missed run is simply skipped.
+          systemd with Persistent=true would catch up at next boot.)
+        - Single-instance guard → launchd refuses to start a second copy of
+          the same Label while one is running (equivalent to the systemd
+          lock-file dance).
+        - "Skip if opencode has never been used" → no native equivalent.
+          mempalace-session exits cleanly (zero drawers filed, fast) when
+          the DB is absent, so no guard is strictly needed. If you want a
+          hard pre-check, wrap ProgramArguments in a shell that tests for
+          ~/Library/Application Support/opencode/opencode.db first.
+    -->
+
+    <key>Label</key>
+    <string>se.jordbo.mempalace-session</string>
+
+    <!-- Absolute path. Install.sh symlinks the wrapper here via cli_utils
+         convention. If you installed mempalace-toolkit somewhere else,
+         update this path accordingly. -->
+    <key>ProgramArguments</key>
+    <array>
+        <string>/Users/USER/.local/bin/mempalace-session</string>
+    </array>
+
+    <!-- launchd gives agents a minimal PATH. Set one that includes the
+         usual macOS locations for brew (Apple Silicon + Intel) plus the
+         user's local bin. mempalace-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, Monday 03:00 local time. Edit to taste:
+           Hour: 0-23  (24-hour)
+           Minute: 0-59
+           Weekday: 0 or 7 = Sunday, 1 = Monday, …, 6 = Saturday
+         Omit a key to match "any". For example: daily 03:00 would use
+         only Hour=3 + Minute=0, no Weekday key. -->
+    <key>StartCalendarInterval</key>
+    <dict>
+        <key>Weekday</key>
+        <integer>1</integer>
+        <key>Hour</key>
+        <integer>3</integer>
+        <key>Minute</key>
+        <integer>0</integer>
+    </dict>
+
+    <!-- Don't run the job just because the plist was loaded. Wait for
+         the StartCalendarInterval trigger. -->
+    <key>RunAtLoad</key>
+    <false/>
+
+    <!-- Background-priority scheduling (CPU + I/O). macOS App Nap and
+         the resource-throttling machinery kick in for ProcessType=Background,
+         so this job yields to interactive work automatically. -->
+    <key>ProcessType</key>
+    <string>Background</string>
+    <key>LowPriorityIO</key>
+    <true/>
+    <key>Nice</key>
+    <integer>10</integer>
+
+    <!-- Runaway guard. Reference 60-session mine takes ~21 min; give 2h.
+         If the process exceeds this, launchd kills it with SIGTERM then
+         SIGKILL. Rare — only matters on pathologically large corpora. -->
+    <key>ExitTimeOut</key>
+    <integer>7200</integer>
+
+    <!-- Logs. Tail with:
+           tail -f ~/Library/Logs/mempalace-session.log
+         Or view in Console.app under "Log Reports". -->
+    <key>StandardOutPath</key>
+    <string>/Users/USER/Library/Logs/mempalace-session.log</string>
+    <key>StandardErrorPath</key>
+    <string>/Users/USER/Library/Logs/mempalace-session.err.log</string>
+</dict>
+</plist>