Acquire and cache opt-in executor capability inventories via V2 (#53736)

## What changed

- Add the disabled-by-default `executor_capability_discovery_v2` feature to acquire `capabilities/discoverV2` inventories from supporting remote executors using each selected environment's working directory and sandbox.
- Share snapshots with MCP and world-state contributors and skill providers without activating discovered plugins or publishing discovered skills to the model.
- Cache inventories by environment instance, executor session, working directory, and sandbox. Coalesce concurrent requests, refresh the cache at turn start, retry transient failures, and refresh MCP projections after recovery.
- Resolve the system skills root in the executor and pass it explicitly to capability location discovery.

## Testing

Add coverage for feature gating, unsupported executors, inert inventories, cache reuse and eviction, turn refreshes, failure recovery, executor replacement, and system skill locations.

GitOrigin-RevId: 1bdca28fa7f17622b27e2713ab3dcc0c13426889

codex-rs/features/src/lib.rs

@@ -275,6 +275,8 @@ pub enum Feature {
     Plugins,
     /// Discover selected-root plugin and skill manifests through one high-level exec-server RPC.
     ExecutorCapabilityDiscovery,
+    /// Acquire V2 executor inventories without publishing or activating their capabilities.
+    ExecutorCapabilityDiscoveryV2,
     /// Skip host skill snapshots when no registered contributor requires them.
     SkipHostSkillDiscovery,
     /// Removed compatibility flag for plugin-bundled lifecycle hooks.
@@ -1628,6 +1630,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::ExecutorCapabilityDiscoveryV2,
+        key: "executor_capability_discovery_v2",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::SkipHostSkillDiscovery,
         key: "skip_host_skill_discovery",