Scope environment readiness config to thread attachments (#38067)

## Why

Threads that share an executor environment can require different capability roots and login-shell policies. Treating readiness configuration as executor-wide can expose one thread's settings to another.

## What changed

- Add `CodexThread::environment_ready` and `EnvironmentConfig` so hosts can install capability roots and login-shell policy for an exact thread, environment, and working-directory attachment.
- Validate attachment readiness updates, merge ready attachment roots with persisted thread roots, and refresh MCP capabilities when those roots change.
- Preserve installed environment policy across thread settings updates and child-thread inheritance while resetting it when the selected working directory changes.

## Testing

- Cover attachment config inheritance and reset behavior.
- Verify that threads sharing an executor retain independent model-visible capability roots and login-shell tool schemas across updates.

GitOrigin-RevId: 60e7a296b3e41b835b0413aa2837451e8a55c230

codex-rs/core-api/src/lib.rs

@@ -27,6 +27,7 @@ pub use codex_config::types::TuiPetAnchor;
 pub use codex_config::types::UriBasedFileOpener;
 pub use codex_core::CodexAppsToolsCache;
 pub use codex_core::CodexThread;
+pub use codex_core::EnvironmentConfig;
 pub use codex_core::ForkSnapshot;
 pub use codex_core::LoadedAgentsMd;
 pub use codex_core::McpManager;
@@ -88,6 +89,8 @@ pub use codex_model_provider_info::built_in_model_providers;
 pub use codex_models_manager::manager::RefreshStrategy;
 pub use codex_models_manager::manager::SharedModelsManager;
 pub use codex_protocol::ThreadId;
+pub use codex_protocol::capabilities::CapabilityRootLocation;
+pub use codex_protocol::capabilities::SelectedCapabilityRoot;
 pub use codex_protocol::config_types::AltScreenMode;
 pub use codex_protocol::config_types::ApprovalsReviewer;
 pub use codex_protocol::config_types::AutoCompactTokenLimitScope;