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/core/src/thread_manager_tests.rs

@@ -687,6 +687,7 @@ async fn start_thread_seeds_extension_data_for_mcp_and_lifecycle_contributors()
             &first_session.services.thread_extension_data,
             &first_originator,
             /*ready_selected_capability_roots*/ &[],
+            /*executor_capability_discovery*/ None,
         )
         .await;
     let second_session = &second_thread.thread.session;
@@ -700,6 +701,7 @@ async fn start_thread_seeds_extension_data_for_mcp_and_lifecycle_contributors()
             &second_session.services.thread_extension_data,
             &second_originator,
             /*ready_selected_capability_roots*/ &[],
+            /*executor_capability_discovery*/ None,
         )
         .await;