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/exec-server/src/server/file_system_handler.rs

@@ -4,6 +4,8 @@ use base64::Engine as _;
 use base64::engine::general_purpose::STANDARD;
 use codex_exec_server_protocol::JSONRPCErrorError;
 
+use crate::CapabilityRootsDiscoverParams;
+use crate::CapabilityRootsDiscoverResponse;
 use crate::CopyOptions;
 use crate::CreateDirectoryOptions;
 use crate::ExecServerRuntimePaths;
@@ -65,6 +67,15 @@ impl FileSystemHandler {
         self.file_reads.close_all().await;
     }
 
+    pub(crate) async fn discover_capability_roots(
+        &self,
+        params: CapabilityRootsDiscoverParams,
+    ) -> Result<CapabilityRootsDiscoverResponse, JSONRPCErrorError> {
+        crate::discover_capability_roots(&self.file_system, params)
+            .await
+            .map_err(|error| invalid_request(error.to_string()))
+    }
+
     pub(crate) async fn open(
         &self,
         params: FsOpenParams,

codex-rs/exec-server/src/server/handler.rs

@@ -14,6 +14,8 @@ use tokio_util::task::TaskTracker;
 use crate::ExecServerRuntimePaths;
 use crate::client::http_client::PendingReqwestHttpBodyStream;
 use crate::client::http_client::ReqwestHttpRequestRunner;
+use crate::protocol::CapabilityRootsDiscoverParams;
+use crate::protocol::CapabilityRootsDiscoverResponse;
 use crate::protocol::EnvironmentInfo;
 use crate::protocol::EnvironmentStatus;
 use crate::protocol::EnvironmentStatusKind;
@@ -258,6 +260,14 @@ impl ExecServerHandler {
         self.file_system.read_file(params).await
     }
 
+    pub(crate) async fn discover_capability_roots(
+        &self,
+        params: CapabilityRootsDiscoverParams,
+    ) -> Result<CapabilityRootsDiscoverResponse, JSONRPCErrorError> {
+        self.require_initialized_for("capability discovery")?;
+        self.file_system.discover_capability_roots(params).await
+    }
+
     pub(crate) async fn fs_open(
         &self,
         params: FsOpenParams,

codex-rs/exec-server/src/server/registry.rs

@@ -1,5 +1,7 @@
 use std::sync::Arc;
 
+use crate::protocol::CAPABILITY_ROOTS_DISCOVER_METHOD;
+use crate::protocol::CapabilityRootsDiscoverParams;
 use crate::protocol::ENVIRONMENT_INFO_METHOD;
 use crate::protocol::ENVIRONMENT_STATUS_METHOD;
 use crate::protocol::EXEC_METHOD;
@@ -76,6 +78,12 @@ pub(crate) fn build_router() -> RpcRouter<ExecServerHandler> {
         ENVIRONMENT_STATUS_METHOD,
         |handler: Arc<ExecServerHandler>, _params: ()| async move { handler.environment_status() },
     );
+    router.request(
+        CAPABILITY_ROOTS_DISCOVER_METHOD,
+        |handler: Arc<ExecServerHandler>, params: CapabilityRootsDiscoverParams| async move {
+            handler.discover_capability_roots(params).await
+        },
+    );
     router.request(
         EXEC_READ_METHOD,
         |handler: Arc<ExecServerHandler>, params: ReadParams| async move {