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/src/lib.rs

@@ -32,6 +32,7 @@ pub use codex_thread::CodexThreadSettingsOverrides;
 pub use codex_thread::ThreadConfigSnapshot;
 pub use codex_thread::TryStartTurnIfIdleError;
 pub use codex_thread::TryStartTurnIfIdleRejectionReason;
+pub use environment_config::EnvironmentConfig;
 pub use session::turn_context::TurnContext;
 pub use user_message_admission::UserMessageAdmission;
 pub use user_message_admission::UserMessageAdmissionError;
@@ -46,6 +47,7 @@ pub mod context;
 mod context_manager;
 mod current_time;
 mod elicitation;
+mod environment_config;
 mod environment_selection;
 pub mod exec;
 pub mod exec_env;