Add turn-scoped environment selections (#18416)

## Summary
- add experimental turn/start.environments params for per-turn
environment id + cwd selections
- pass selections through core protocol ops and resolve them with
EnvironmentManager before TurnContext creation
- treat omitted selections as default behavior, empty selections as no
environment, and non-empty selections as first environment/cwd as the
turn primary

## Testing
- ran `just fmt`
- ran `just write-app-server-schema`
- not run: unit tests for this stacked PR

---------

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

codex-rs/app-server/README.md

@@ -533,6 +533,10 @@ You can optionally specify config overrides on the new turn. If specified, these
     "input": [ { "type": "text", "text": "Run tests" } ],
     // Below are optional config overrides
     "cwd": "/Users/me/project",
+    // Experimental: turn-scoped environment selection.
+    "environments": [
+        { "environmentId": "local", "cwd": "/Users/me/project" }
+    ],
     "approvalPolicy": "unlessTrusted",
     "sandboxPolicy": {
         "type": "workspaceWrite",