feat(base)!: removing a profile makes it private (ADR 0020, D27)

Every account keeps a handle now, so removing a public profile no
longer deletes it and frees the handle: it takes away what the profile
shows and leaves it private, as a profile is until its person goes
public (D5).

- `profile.remove` and the register's `removeProfile` (members ask the
  register) empty the display name, bio, links with their checks and
  the avatar, keep the row with its handle, and return what is left,
  null for none. `/@handle` and `profiles.byHandle` then answer as for
  a handle nobody has. Once the person has published, removing is
  refused as before.
- Learn: the profile form starts afresh from what is left, "Remove
  profile" shows only while something is filled in, and the dialog says
  the handle stays.
- The module's README and ADR 0020 amended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/test/integration/register/register_test.dart

@@ -318,6 +318,22 @@ void main() {
             refused(ProfileRefusal.handleTaken),
           );
 
+          await remote.saveProfile(
+            session,
+            anna,
+            handle: 'Anna',
+            displayName: 'Anna',
+            bio: '',
+            links: const [],
+          );
+          final removed = (await remote.removeProfile(session, anna))!;
+          expect(
+            (removed.handle, removed.displayName),
+            ('Anna', ''),
+            reason: 'removing keeps the handle, private (ADR 0020, D27)',
+          );
+          expect(await remote.removeProfile(session, ben), isNull);
+
           await remote.fixHandle(session, anna);
 
           expect((await remote.profileOf(session, anna))!.handleFixed, isTrue);