Pair thread environment settings (#26687)

## Why

Thread cwd and environment selections are a single logical setting in
core: updating one without the other can silently desynchronize the
next-turn execution context. This change makes that relationship
explicit in the internal thread settings flow while preserving the
existing app-server public API shape.

## What changed

- Moved the cwd/environment pair through internal
`ThreadSettingsOverrides.environment_settings` instead of a top-level
internal `cwd` field.
- Kept `thread/settings/update` public params unchanged, with app-server
translating top-level `cwd` into the paired internal settings shape.
- Moved `Op::UserInput` environment overrides into thread settings so
user turns and settings updates use the same core path.
- Updated core, app-server, MCP, memories, sample, and test callsites to
construct the paired settings shape.

## Verification

- `git diff --check`
- Local test run starting after PR creation.

codex-rs/core/src/codex_thread.rs

@@ -32,6 +32,7 @@ use codex_protocol::protocol::ThreadMemoryMode;
 use codex_protocol::protocol::ThreadSource;
 use codex_protocol::protocol::TokenUsageInfo;
 use codex_protocol::protocol::TurnEnvironmentSelection;
+use codex_protocol::protocol::TurnEnvironmentSelections;
 use codex_protocol::protocol::W3cTraceContext;
 use codex_protocol::user_input::UserInput;
 use codex_thread_store::StoredThread;
@@ -59,7 +60,7 @@ pub struct ThreadConfigSnapshot {
     pub approvals_reviewer: ApprovalsReviewer,
     pub permission_profile: PermissionProfile,
     pub active_permission_profile: Option<ActivePermissionProfile>,
-    pub cwd: AbsolutePathBuf,
+    pub environments: TurnEnvironmentSelections,
     pub workspace_roots: Vec<AbsolutePathBuf>,
     pub profile_workspace_roots: Vec<AbsolutePathBuf>,
     pub ephemeral: bool,
@@ -114,10 +115,18 @@ impl TryStartTurnIfIdleError {
 }
 
 impl ThreadConfigSnapshot {
+    pub fn cwd(&self) -> &AbsolutePathBuf {
+        &self.environments.legacy_fallback_cwd
+    }
+
+    pub fn environment_selections(&self) -> &[TurnEnvironmentSelection] {
+        &self.environments.environments
+    }
+
     pub fn sandbox_policy(&self) -> SandboxPolicy {
         codex_sandboxing::compatibility_sandbox_policy_for_permission_profile(
             &self.permission_profile,
-            self.cwd.as_path(),
+            self.cwd().as_path(),
         )
     }
 }
@@ -125,7 +134,7 @@ impl ThreadConfigSnapshot {
 /// Thread settings overrides that app-server validates before starting a turn.
 #[derive(Clone, Default)]
 pub struct CodexThreadSettingsOverrides {
-    pub cwd: Option<AbsolutePathBuf>,
+    pub environments: Option<TurnEnvironmentSelections>,
     pub workspace_roots: Option<Vec<AbsolutePathBuf>>,
     pub profile_workspace_roots: Option<Vec<AbsolutePathBuf>>,
     pub approval_policy: Option<AskForApproval>,
@@ -330,7 +339,7 @@ impl CodexThread {
         overrides: CodexThreadSettingsOverrides,
     ) -> SessionSettingsUpdate {
         let CodexThreadSettingsOverrides {
-            cwd,
+            environments,
             workspace_roots,
             profile_workspace_roots,
             approval_policy,
@@ -357,7 +366,7 @@ impl CodexThread {
         };
 
         SessionSettingsUpdate {
-            cwd,
+            environments,
             workspace_roots,
             profile_workspace_roots,
             approval_policy,