Activate selected executor plugin MCPs in app-server (#27893)

## Why

#27870 teaches the MCP extension how to discover stdio MCP servers
declared by a selected executor plugin, but app-server does not yet
install that contributor or initialize its per-thread state. As a
result, `thread/start.selectedCapabilityRoots` can select the plugin
while its MCP servers remain inactive.

This PR closes that app-server wiring gap:

```text
thread/start(selectedCapabilityRoots)
    -> initialize the thread's selected-plugin MCP snapshot
    -> read the selected plugin's .mcp.json through its environment
    -> start declared stdio servers in that environment
    -> expose their tools only on the selected thread
```

## What changed

- Install the selected-executor-plugin MCP contributor in app-server
using the existing shared `EnvironmentManager`.
- Initialize its frozen thread snapshot when `thread/start` includes
selected capability roots.
- Document that selected plugin stdio MCPs are activated in their owning
environment.
- Add an app-server E2E covering the complete selection-to-tool-call
path.

The E2E verifies that:

- the selected MCP process receives an executor-only environment value,
proving the tool runs through the selected environment;
- the MCP tool is advertised to the model and can be called;
- a normal MCP config reload does not discard the thread's frozen
selected-plugin registration;
- another thread without the selected root does not see the MCP server.

## Scope

- Existing sessions without `selectedCapabilityRoots` are unchanged.
- Only stdio MCP declarations are activated. HTTP declarations remain
inactive.
- This does not change selected-root persistence across resume/fork or
add hosted-plugin behavior.

## Verification

- Focused app-server E2E:
`selected_executor_plugin_exposes_its_stdio_mcp_only_to_that_thread`

## Stack

Stacked on #27870.

codex-rs/app-server/src/extensions.rs

@@ -9,6 +9,7 @@ use codex_core::NewThread;
 use codex_core::StartThreadOptions;
 use codex_core::ThreadManager;
 use codex_core::config::Config;
+use codex_exec_server::EnvironmentManager;
 use codex_extension_api::AgentSpawnFuture;
 use codex_extension_api::AgentSpawner;
 use codex_extension_api::ExtensionEventSink;
@@ -27,9 +28,6 @@ use crate::outgoing_message::OutgoingMessageSender;
 use crate::thread_state::ThreadListenerCommand;
 use crate::thread_state::ThreadStateManager;
 
-// TODO(jif): Enable once /ps/mcp serves complete hosted skill packages.
-const ORCHESTRATOR_SKILLS_ENABLED: bool = false;
-
 pub(crate) struct ThreadExtensionDependencies {
     pub(crate) event_sink: Arc<dyn ExtensionEventSink>,
     pub(crate) auth_manager: Arc<AuthManager>,
@@ -37,6 +35,7 @@ pub(crate) struct ThreadExtensionDependencies {
     pub(crate) analytics_events_client: AnalyticsEventsClient,
     pub(crate) thread_manager: Weak<ThreadManager>,
     pub(crate) goal_service: Arc<GoalService>,
+    pub(crate) environment_manager: Arc<EnvironmentManager>,
     pub(crate) executor_skill_provider: Arc<dyn codex_skills_extension::SkillProvider>,
     /// Process-scoped persistence backend for extensions that need stored thread history.
     pub(crate) thread_store: Arc<dyn ThreadStore>,
@@ -56,6 +55,7 @@ where
         analytics_events_client,
         thread_manager,
         goal_service,
+        environment_manager,
         executor_skill_provider,
         thread_store: _thread_store,
     } = dependencies;
@@ -74,15 +74,14 @@ where
     codex_guardian::install(&mut builder, guardian_agent_spawner);
     codex_memories_extension::install(&mut builder, codex_otel::global());
     codex_mcp_extension::install(&mut builder);
+    codex_mcp_extension::install_executor_plugins(&mut builder, environment_manager);
     codex_web_search_extension::install(&mut builder, auth_manager.clone());
     codex_image_generation_extension::install(&mut builder, auth_manager);
-    let mut skill_providers = codex_skills_extension::SkillProviders::new()
-        .with_executor_provider(executor_skill_provider);
-    if ORCHESTRATOR_SKILLS_ENABLED {
-        skill_providers = skill_providers.with_orchestrator_provider(Arc::new(
+    let skill_providers = codex_skills_extension::SkillProviders::new()
+        .with_executor_provider(executor_skill_provider)
+        .with_orchestrator_provider(Arc::new(
             codex_skills_extension::OrchestratorSkillProvider::new(),
         ));
-    }
     codex_skills_extension::install_with_providers(
         &mut builder,
         skill_providers,