contrib: flag cron-not-installed as a common caveat

Minimal Debian/Ubuntu hosts (and most base container images) don't
ship cron by default. `crontab: command not found` is the first
thing a user hits if they try the cron path without installing it.
Previous caveats block covered semantics (no Persistent, mail-drop
stderr) but silently assumed cron was present. Add an explicit
"check command -v crontab, apt install cron, or pick systemd"
preflight to the caveats so the error is surfaced before the
user runs into it.

Caught during 2026-04-30 Phase 4 runtime validation on a Debian
trixie host: `crontab -T` lint failed because cron wasn't
installed, even though the underlying docker-exec shell command
(the actual workload) ran fine.

contrib/README.md

@@ -140,7 +140,7 @@ rm ~/Library/LaunchAgents/se.jordbo.mempalace-session.plist
 
 **Why:** simpler, ubiquitous, works on any UNIX. No `loginctl enable-linger` dance, no user-units awareness required.
 
-**Caveats:** no "persistent" semantics (a missed run while the machine was off stays missed); default cron output goes to mail or is silently dropped if no MTA.
+**Caveats:** no "persistent" semantics (a missed run while the machine was off stays missed); default cron output goes to mail or is silently dropped if no MTA. Also: **cron is not installed by default on minimal Debian/Ubuntu hosts** (nor on most container images). Check with `command -v crontab` — if absent, `sudo apt install cron` (or equivalent) first, or use the systemd timer instead.
 
 **Install:**