feat: deferred items — arena token API + UI, ToolShim docs

- Arena API: token_breakdown selected in contestant query
- ArenaPane: token category breakdown bar (s/u/a/t/r) in expanded contestant view
- apps/server/CLAUDE.md: document tool-shim and loop-detectors

apps/coder/src/routes/arena.ts

@@ -205,7 +205,7 @@ export function registerArenaRoutes(
 
     const contestants = await sql`
       SELECT id, battle_id, identity, model, lane, task_id, worktree_id,
-             status, duration_ms, tokens_per_sec, cost_tokens, result_path, error,
+             status, duration_ms, tokens_per_sec, cost_tokens, token_breakdown, result_path, error,
              created_at, updated_at
       FROM contestants
       WHERE battle_id = ${id}