Prepare selected environment plumbing (#20669)

## Why
This is a prep PR in the multi-environment process-tool stack. It
separates ownership/config cleanup from the behavior change that teaches
process tools to route by selected environment, so the follow-up PR can
focus on model-facing `environment_id` behavior.

## Stack
1. https://github.com/openai/codex/pull/20646 - `EnvironmentContext`
rendering for selected environments
2. https://github.com/openai/codex/pull/20669 - selected-environment
ownership and tool config prep (this PR)
3. https://github.com/openai/codex/pull/20647 - process-tool
`environment_id` routing

## What Changed
- keep the resolved turn environment list wrapped in
`ResolvedTurnEnvironments` through `TurnContext` instead of unwrapping
it back to a raw `Vec`
- add `TurnContext::resolve_path_against` so cwd-relative path
resolution has one shared helper
- replace the old tool config boolean with `ToolEnvironmentMode::{None,
Single, Multiple}`

## Testing
- Tests not run locally; this prep refactor is covered by GitHub CI for
the stack.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/environment_selection.rs

@@ -24,7 +24,7 @@ pub(crate) fn default_thread_environment_selections(
         .collect()
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Default)]
 pub(crate) struct ResolvedTurnEnvironments {
     pub(crate) turn_environments: Vec<TurnEnvironment>,
 }
@@ -37,17 +37,17 @@ impl ResolvedTurnEnvironments {
             .collect()
     }
 
-    pub(crate) fn primary_turn_environment(&self) -> Option<&TurnEnvironment> {
+    pub(crate) fn primary(&self) -> Option<&TurnEnvironment> {
         self.turn_environments.first()
     }
 
     pub(crate) fn primary_environment(&self) -> Option<Arc<codex_exec_server::Environment>> {
-        self.primary_turn_environment()
+        self.primary()
             .map(|environment| Arc::clone(&environment.environment))
     }
 
     pub(crate) fn primary_filesystem(&self) -> Option<Arc<dyn ExecutorFileSystem>> {
-        self.primary_turn_environment()
+        self.primary()
             .map(|environment| environment.environment.get_filesystem())
     }
 }
@@ -171,7 +171,7 @@ mod tests {
 
         assert_eq!(
             resolved
-                .primary_turn_environment()
+                .primary()
                 .expect("primary environment")
                 .environment_id,
             "local"