fix(rooms): default new rooms to tribunal when room_mode is omitted

assign_room (MCP tool + host-side assignRoomInDatabase) and setup
register previously fell back to 'single' when no room_mode was given,
so newly added rooms silently lost the reviewer. Default to 'tribunal'
across the MCP zod schema, the IPC arg fallback, and the DB helper, and
update the ipc-auth expectation accordingly.

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

runners/agent-runner/src/ipc-mcp-stdio.ts

@@ -765,8 +765,10 @@ If folder is omitted, the host generates one automatically.`,
     name: z.string().describe('Display name for the room'),
     room_mode: z
       .enum(['single', 'tribunal'])
-      .default('single')
-      .describe('single=owner only, tribunal=owner/reviewer roles enabled'),
+      .default('tribunal')
+      .describe(
+        'single=owner only, tribunal=owner/reviewer roles enabled (default)',
+      ),
     owner_agent_type: z
       .enum(['claude-code', 'codex'])
       .optional()
@@ -831,7 +833,7 @@ If folder is omitted, the host generates one automatically.`,
       type: 'assign_room',
       jid: args.jid,
       name: args.name,
-      room_mode: args.room_mode || 'single',
+      room_mode: args.room_mode || 'tribunal',
       owner_agent_type: args.owner_agent_type,
       reviewer_agent_type: args.reviewer_agent_type,
       arbiter_agent_type: args.arbiter_agent_type,