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-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -14154,6 +14154,21 @@
       "title": "TurnDiffUpdatedNotification",
       "type": "object"
     },
+    "TurnEnvironmentParams": {
+      "properties": {
+        "cwd": {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        },
+        "environmentId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "cwd",
+        "environmentId"
+      ],
+      "type": "object"
+    },
     "TurnError": {
       "properties": {
         "additionalDetails": {