[codex] retain resolved environments across turns (#27955)

## Why

Selected execution environments are thread-scoped resources, but startup
and turn construction repeatedly resolved their IDs and working
directories. That discarded existing environment handles and shell
metadata even when a selection had not changed.

Session configuration updates also need to affect future turns without
changing the resolved environment set already captured by a running
turn.

## What changed

- Create a `ThreadEnvironments` service inside `Codex` from the spawned
`EnvironmentManager` and raw environment selections, then store it on
`SessionServices`.
- Split service construction from `update_selections`, allowing session
configuration updates to mutate the resolved set in place.
- Retain an existing `TurnEnvironment` when its environment ID and
working directory match; resolve only added or changed selections and
remove selections that are no longer present.
- Normalize duplicate IDs by keeping the first selection and skip
individual selections that fail to resolve instead of rejecting the
entire update.
- Give each `TurnContext` a cloned `TurnEnvironmentSnapshot`, so later
session configuration updates affect future turns without rewriting an
active turn.
- Reuse the service-owned environment manager and resolved snapshot for
startup work, MCP initialization, and child-thread spawning instead of
flowing resolved environments through spawn arguments.

## Test plan

- `cargo check -p codex-core --tests`
- `just test -p codex-core environment_selection`
- `just test -p codex-core turn_environments`
- `just test -p codex-core
session_update_settings_does_not_rewrite_sticky_environment_cwds`
- `just test -p codex-core
default_turn_does_not_overlay_legacy_fallback_cwd_onto_stored_thread_environments`

codex-rs/core/src/agents_md.rs

@@ -18,7 +18,7 @@
 use crate::config::Config;
 use crate::context::ContextualUserFragment;
 use crate::context::UserInstructions as ContextUserInstructions;
-use crate::environment_selection::ResolvedTurnEnvironments;
+use crate::environment_selection::TurnEnvironmentSnapshot;
 use codex_app_server_protocol::ConfigLayerSource;
 use codex_config::ConfigLayerStackOrdering;
 use codex_config::default_project_root_markers;
@@ -48,7 +48,7 @@ const AGENTS_MD_SEPARATOR: &str = "\n\n--- project-doc ---\n\n";
 pub(crate) async fn load_project_instructions(
     config: &mut Config,
     user_instructions: Option<UserInstructions>,
-    environments: &ResolvedTurnEnvironments,
+    environments: &TurnEnvironmentSnapshot,
 ) -> Option<LoadedAgentsMd> {
     let mut loaded = LoadedAgentsMd::from_user_instructions(user_instructions);
     for turn_environment in &environments.turn_environments {