docs(admin): on-site notifications, and the defaults page that was wrong

Engagement Phase 7 gives the platform a third notification channel — an inbox
on the site itself — so `notifications-and-email.mdx` gains a section for it:
what it is, that it is the one channel on by default, that its body is always
plain text, and that the nightly prune takes read items only.

Two corrections in the same file, both of which were already false before this
phase and would have become misleading with it:

- "Who receives what" said push was opt-OUT. Push stream subscriptions have
  always been opt-IN, and engagement Phase 3 made that explicit in the channel
  registry. Rewritten as three defaults plus the Team mute that overrides all
  three, and pointed at the preferences grid the same phase gave the web.
- `capabilities.mjs` claimed "Web, push and email … push arrives by default".
  The web channel did not exist until now and push has never arrived by
  default. Reworded to name the on-site inbox as the opt-out one.

The push section now says what a tickle raised by an engagement rule carries,
and that it is still only a pointer.

Code: RunicGateway/website#TBD · Docs: RunicGateway/docs#TBD

Co-Authored-By: Claude <noreply@anthropic.com>

src/data/capabilities.mjs

@@ -139,9 +139,9 @@ const community = {
     {
       label: 'Notifications',
       detail:
-        'Web, push and email, chosen per stream by each person rather than per person by ' +
-        'you. Push arrives by default and can be switched off; email only ever arrives if ' +
-        'it was asked for.',
+        'On the site, by push and by email, chosen per notification by each person rather ' +
+        'than per person by you. The on-site inbox arrives by default and can be switched ' +
+        'off; push and email only ever arrive if they were asked for.',
     },
     {
       label: 'Wiki',