Add an opt-in feature for subagent model context defaults (#52659)

## What changed

Add the disabled-by-default `subagent_default_context_limits` feature and expose it in the configuration schema. When enabled, shared configuration for spawning and resuming subagents clears inherited `model_context_window` and `model_auto_compact_token_limit` overrides so subagents use their model's default context window and auto-compaction threshold.

GitOrigin-RevId: 5168ae2b4fbe72c533d8c6152922a8bceb2c5cbd

codex-rs/features/src/lib.rs

@@ -225,6 +225,8 @@ pub enum Feature {
     ModelCatalogInContext,
     /// Inherit client-defined dynamic tools in fresh V2 subagents.
     MultiAgentV2DynamicTools,
+    /// Use each subagent model's default context window and auto-compaction threshold.
+    SubagentDefaultContextLimits,
     /// Keep sampling through reasoning and commentary boundaries when agent mail arrives.
     /// Pending mail is delivered at the next normal input boundary instead.
     DeferMailboxPreemption,
@@ -1464,6 +1466,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::SubagentDefaultContextLimits,
+        key: "subagent_default_context_limits",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::DeferMailboxPreemption,
         key: "defer_mailbox_preemption",