docs: organize diagrams to mirror code layout (util/ + features/)

Move flat docs/diagrams/*.puml into a hierarchy matching the source
package structure:

  docs/diagrams/
  ├── bootstrap-sequence.puml         (cross-cutting)
  ├── events-diagram.puml             (cross-feature)
  ├── util/
  │   ├── command-class-diagram.puml
  │   ├── database-diagram.puml
  │   ├── messages-class-diagram.puml
  │   ├── broadcasts-class-diagram.puml
  │   └── gui-class-diagram.puml
  └── features/
      ├── team/
      │   ├── team-class-diagram.puml
      │   ├── team-config-class-diagram.puml
      │   ├── builtin-commands-diagram.puml
      │   ├── team-create-sequence.puml
      │   ├── team-create-activity.puml
      │   └── team-join-sequence.puml
      ├── player/
      │   └── player-class-diagram.puml
      └── moderation/
          └── moderation-class-diagram.puml

README.md diagram index split into 4 sections (overview, util,
features/team, features/player, features/moderation) for readability;
all links updated. features.md auto-updated by sed for the new paths.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/diagrams/features/team/team-create-activity.puml

@@ -0,0 +1,40 @@
+@startuml team-create-activity
+title CR-Core — Create Team (activity diagram)
+
+start
+
+:Player runs /team create <name> <tag> <color> [visibility];
+
+if (Name already in use?) then (yes)
+  :Reply "team name already taken";
+  stop
+else (no)
+endif
+
+if (Tag already in use?) then (yes)
+  :Reply "team tag already taken";
+  stop
+else (no)
+endif
+
+if (Player already in a team?) then (yes)
+  :Reply "you already belong to a team";
+  stop
+else (no)
+endif
+
+if (Color valid?) then (no)
+  :Reply "unknown color";
+  stop
+else (yes)
+endif
+
+:Create Team(id = randomUUID, name, tag, color, leaderId = playerId, visibility);
+note right: visibility defaults to PRIVATE\nif not specified
+:Add player as TeamMember(role = LEADER);
+:Persist via TeamRepository.save(team);
+:Reply "team created";
+
+stop
+
+@enduml