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.

SKILL.md

@@ -141,10 +141,11 @@ A machine running only one of these has half a memory. Agents loading the `mempa
 
 ### Quick automation pitch
 
-If the user has a devbox and hasn't set up automation, offer it:
+If the user has a devbox and hasn't set up automation, offer it. Pick the scheduler that matches the OS:
+
+**Linux (systemd user timer):**
 
 ```bash
-# One-time setup (modern Linux, systemd user timer)
 cd ~/mempalace-toolkit
 mkdir -p ~/.config/systemd/user
 cp contrib/systemd/*.{service,timer} ~/.config/systemd/user/
@@ -153,7 +154,20 @@ systemctl --user enable --now mempalace-session.timer
 sudo loginctl enable-linger "$USER"   # optional, for headless boxes
 ```
 
-Full install/verify/uninstall recipes for both systemd and cron are in [`contrib/README.md`](../../contrib/README.md). The quick-start above defaults to weekly runs on Monday 03:00 local time with a ≤30 min randomized delay.
+**macOS (launchd user agent):**
+
+```bash
+cd ~/mempalace-toolkit
+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"
+```
+
+**BSD or systemd-less Linux (cron):** see [`contrib/cron/`](../../contrib/cron/).
+
+Full install/verify/uninstall recipes for all three are in [`contrib/README.md`](../../contrib/README.md). All three default to weekly runs on Monday 03:00 local time.
 
 ## Failure Modes & Fixes