build: people: memberships - invite by phone number, list who is in

With portal keeping the groups itself (portal v0.5.7), a desk's invite
form asks for a phone number first, the list of invites is the list
of memberships, and a grant may know the person by number.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GT37Z1Xtfd9pUuQtMTg6Yt

src/plan.rs

@@ -169,6 +169,12 @@ rest. Each is one line in the plan, and the builder does the wiring:
   behind sign-in, with name and address filled from the account: a
   seat on a committee for members, a discount for customers. The
   group is one a decision `grants`, so nobody has to be waved in.
+- **Keep the people in portal.** `people: memberships` keeps every
+  group in portal itself: a desk invite is a place held until the
+  person signs in, a person is their phone number first, and any
+  sign-in provider that says who someone is will do (Vipps, for a
+  Norwegian association). Invite forms then ask for a phone number.
+  Without it, Kanidm makes an account per invite.
 - **Answer for the pages as a group.** `contact: { name, email, group }`
   makes the contact a group's inbox rather than a person: no account
   is made for it, and every member of the group may suggest changes