Add batched executor capability discovery (#33852)

## Why

Selected capability roots can contribute plugins, MCP servers, connectors, and
skills. Discovering each contribution separately requires repeated access to the
executor filesystem.

## What changed

- Add the `capabilityRoots/discoverV1` exec-server RPC to scan selected roots and
  materialize recognized plugin manifests, configuration files, skill
  instructions, and skill metadata in one bounded request.
- Add the opt-in `executor_capability_discovery` feature, with a thread-scoped
  cache and per-step snapshot shared by MCP and skill discovery.
- Parse MCP, connector, and skill contributions from the materialized snapshot,
  including serving cached skill instructions without another filesystem read.

## Testing

- Cover discovery limits, manifest precedence, root-local failures, cache reuse,
  plugin contributions, and parity with the existing environment skill loader.

GitOrigin-RevId: f98fd2321cafb58c596db02da1f83c09d8eb375d

codex-rs/features/src/tests.rs

@@ -27,6 +27,19 @@ fn under_development_features_are_disabled_by_default() {
     }
 }
 
+#[test]
+fn executor_capability_discovery_is_an_opt_in_map_feature() {
+    let mut features = Features::with_defaults();
+    assert!(!features.enabled(Feature::ExecutorCapabilityDiscovery));
+
+    features.apply_map(&BTreeMap::from([(
+        "executor_capability_discovery".to_string(),
+        true,
+    )]));
+
+    assert!(features.enabled(Feature::ExecutorCapabilityDiscovery));
+}
+
 #[test]
 fn default_enabled_features_are_stable() {
     for spec in crate::FEATURES {