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/session/mod.rs

@@ -205,6 +205,7 @@ use codex_protocol::exec_output::StreamOutput;
 
 mod code_mode_warning;
 pub(crate) mod context_window;
+mod environment;
 mod extension_metrics;
 mod handlers;
 mod inject;