Revert the showmissed lock of #277: the operator wants no special handling in betterbird's config (hq issue 640)

The fix is the approved settings layer (proposal s09bf8c99da7ac8b9:
calendar.alarms.showmissed false), pushed to g14 and shanks.

modules/betterbird/README.md

@@ -97,7 +97,7 @@ start; `prefs.js` wins over a default.
 value true, false, a whole number or a string. The node-engine merges the layers key by key, so a node's layer
 overrides one preference, or takes one out with `null`. The operator's setup approved on 2026-10-12 (compact
 density, 14-point text, the vertical view with multi-line cards and threads, no new-mail pop-up, sound or tray
-icon, the calendar's day from 09:00 to 20:00, reminders missed while asleep shown (since locked off: issue 640), chat off, remote content
+icon, the calendar's day from 09:00 to 20:00, reminders missed while asleep shown, chat off, remote content
 blocked, Do Not Track; 21 preferences) moved from the catalogue's defaults file into the mesh-wide layer of
 `prefs`, set on the operator's Approve.
 
@@ -109,24 +109,10 @@ desktop node writes the map's text into the old build's user.js, and `betterbird
 refused until the push.
 
 **Locked** (the policy): Betterbird's updater off (`DisableAppUpdate`); telemetry off (`DisableTelemetry`);
-add-ons change only through the mesh's pins (`ExtensionUpdate` false); and four preferences locked off:
+add-ons change only through the mesh's pins (`ExtensionUpdate` false); and three preferences locked off:
 `mail.shell.checkDefaultClient` (the ask to be the default client), `mail.inappnotifications.enabled` (the
-vendor's in-app notices), `mailnews.start_page.enabled` (the start page, which otherwise loads from the
-vendor at every start), and `calendar.alarms.showmissed` (the reminders of meetings that have passed, below).
-
-**A passed meeting's reminder is not shown again** (novox/hq issue 640). With `calendar.alarms.showmissed` on,
-Betterbird's alarm service fires, at every load of a calendar (each start, each refresh, and each time Owl
-registers its calendar after signing in), every reminder of the past month whose item carries no later
-acknowledgement (`X-MOZ-LASTACK`). Off, Betterbird's own default, a past reminder fires only while its meeting
-has not ended. A dismissal is that acknowledgement, and Owl never sends it to Exchange (its changes carry the
-reminder's offset alone): it lives only in Betterbird's local calendar cache, which Owl empties when it
-resynchronises a calendar from scratch. With the pass on, Exchange meetings of the past month came back about
-every ten minutes on g14 from the start that placed the setting's `true`, Dismiss or not. So the policy locks the
-pass off, whatever the setting `prefs` or a profile says; a setting's value for it has no effect (`betterbird_config`
-shows the lock). What it costs: a reminder that fell due while the laptop slept is not shown once the meeting
-is over; one for a meeting still running is. `betterbird_check` says when the running Betterbird still has the
-pass on beside an Owl calendar, with the remedy (the module's push, or `betterbird_restart` once the policy is
-placed).
+vendor's in-app notices), and `mailnews.start_page.enabled` (the start page, which otherwise loads from the
+vendor at every start).
 
 **Kept at Betterbird's default** (`// keeps Betterbird's default: <name>` in `mesh-defaults.js`): the theme,
 system colours, always-underlined links, manual spam marking, and the composer's attachment pane. A value the