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-join-sequence.puml

@@ -0,0 +1,62 @@
+@startuml team-join-sequence
+title CR-Core — Player joins a public team (sequence diagram)
+
+actor Player
+participant "BaseCommand\n(TeamCommand)" as Base
+participant "SubCommand\n(TeamJoinSub)" as Sub
+participant "TeamService" as Service
+participant "TeamRepository" as Repo
+participant "Team" as Team
+
+Player -> Base : /team join <name>
+activate Base
+
+Base -> Base : route args[0]="join" → TeamJoinSub
+Base -> Sub : execute(ctx)
+activate Sub
+
+Sub -> Service : getTeamByName(name)
+activate Service
+Service -> Repo : findByName(name)
+activate Repo
+Repo --> Service : Optional<Team>
+deactivate Repo
+Service --> Sub : Optional<Team>
+deactivate Service
+
+alt team not found
+  Sub --> Base : CommandResult.failure("Team not found")
+else team found
+  Sub -> Service : joinTeam(team.id, playerId)
+  activate Service
+
+  Service -> Service : validateJoinable(team, playerId)
+  alt team is PRIVATE
+    Service --> Sub : throw TeamAccessException
+    Sub --> Base : CommandResult.failure(ex.message)
+  else player already in a team
+    Service --> Sub : throw TeamAccessException
+    Sub --> Base : CommandResult.failure(ex.message)
+  else allowed
+    Service -> Team : addMember(playerId)
+    activate Team
+    Team --> Service : member
+    deactivate Team
+    Service -> Repo : save(team)
+    activate Repo
+    Repo --> Service : team
+    deactivate Repo
+    Service -> Service : onMemberAdded(team, member)
+    Service -> Service : onPlayerJoined(team, member)
+    Service --> Sub : true
+    Sub --> Base : CommandResult.success("Joined " + team.name)
+  end
+  deactivate Service
+end
+
+deactivate Sub
+Base -> Base : handleResult → sender.sendMessage
+Base --> Player : reply
+deactivate Base
+
+@enduml