refactor(x): drop limits DI, read setting at default site; async fs

Address review: remove ITurnLimitsResolver, its bridge and container
registration. createTurn now defaults an omitted maxModelCalls straight
from the turn-limits config (lazy import, built-in fallback), with no
chat/headless distinction in the runtime — the chat UI passes its
optional chat override explicitly via the existing sessions:sendMessage
maxModelCalls field. turn_limits.ts now uses fs/promises throughout.

apps/x/packages/core/docs/turn-runtime-design.md

@@ -512,13 +512,13 @@ Rules:
 - `input` is the user message that defines this turn boundary.
 - `autoPermission` defaults to `false` before persistence.
 - `humanAvailable` is required explicitly.
-- `maxModelCalls`, when omitted, is resolved at creation from the user's
-  turn-limit settings (`config/turn_limits.json`) via the injected
-  `ITurnLimitsResolver`: turns with `humanAvailable: true` use the chat
-  override when set, otherwise the global limit; headless turns always use
-  the global limit. Without a resolver (tests) it defaults to `50`. Spawned
-  sub-agents default to the global limit, which also caps the budget a
-  parent may grant them. Settings changes affect only newly created turns.
+- `maxModelCalls`, when omitted, defaults at creation to the user's global
+  model-call limit (`config/turn_limits.json`, built-in default `50`). The
+  runtime makes no chat/headless distinction: the optional chat override is
+  the chat UI's job — its send path passes an explicit `maxModelCalls`.
+  Spawned sub-agents default to the global limit, which also caps the
+  budget a parent may grant them. Settings changes affect only newly
+  created turns.
 - Persisted values are fully resolved and immutable.
 
 The capability is named `humanAvailable`, not `headless`. `headless` describes