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.

README.md

@@ -74,10 +74,11 @@ mempalace-docs /workspace/my_project
 
 Manual invocation is fine while you're actively driving the machine, but long-running devboxes benefit from a weekly automated mine. [`contrib/`](contrib/) ships ready-to-install templates:
 
-- **systemd user timer** (recommended on modern Linux): survives reboots, catches missed runs, logs to `journalctl`.
-- **cron**: simpler, works everywhere, no user-unit awareness needed.
+- **systemd user timer** (recommended on Linux): survives reboots, catches missed runs, logs to `journalctl`.
+- **launchd user agent** (recommended on macOS): native-equivalent — logs to `~/Library/Logs/`, single-instance guarantees, `ProcessType=Background` throttling.
+- **cron**: simplest, works on BSD and systemd-less distros. No user-unit awareness needed.
 
-Quick-start (systemd, weekly Mon 03:00 local):
+Quick-start (Linux / systemd, weekly Mon 03:00 local):
 
 ```bash
 mkdir -p ~/.config/systemd/user
@@ -87,6 +88,16 @@ systemctl --user enable --now mempalace-session.timer
 sudo loginctl enable-linger "$USER"   # optional, for headless boxes
 ```
 
+Quick-start (macOS / launchd, same schedule):
+
+```bash
+sed "s|USER|$USER|g" contrib/launchd/se.jordbo.mempalace-session.plist \
+  > ~/Library/LaunchAgents/se.jordbo.mempalace-session.plist
+mkdir -p ~/Library/Logs
+launchctl bootstrap "gui/$(id -u)" ~/Library/LaunchAgents/se.jordbo.mempalace-session.plist
+launchctl enable "gui/$(id -u)/se.jordbo.mempalace-session"
+```
+
 See [`contrib/README.md`](contrib/README.md) for full install/verify/uninstall recipes, tuning, and devbox/container caveats. The full operational routine (triggers, cadence, verification) is in [`ARCHITECTURE.md`](ARCHITECTURE.md) §5.
 
 ### Containerized (devbox) notes