app-server: add streaming command/exec contract and implementation

Define the v2 command/exec contract and wire it through app-server and core in one slice: processId, env overrides, timeout and output-cap controls, streaming notifications, and PTY write/resize/terminate support.

Keep the generated schema, README updates, backend plumbing, and test harness changes together so reviewers can read the API and runtime behavior in the same commit.

codex-rs/app-server-protocol/schema/json/v2/CommandExecResizeParams.json

@@ -0,0 +1,38 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "CommandExecTerminalSize": {
+      "properties": {
+        "cols": {
+          "format": "uint16",
+          "minimum": 0.0,
+          "type": "integer"
+        },
+        "rows": {
+          "format": "uint16",
+          "minimum": 0.0,
+          "type": "integer"
+        }
+      },
+      "required": [
+        "cols",
+        "rows"
+      ],
+      "type": "object"
+    }
+  },
+  "properties": {
+    "processId": {
+      "type": "string"
+    },
+    "size": {
+      "$ref": "#/definitions/CommandExecTerminalSize"
+    }
+  },
+  "required": [
+    "processId",
+    "size"
+  ],
+  "title": "CommandExecResizeParams",
+  "type": "object"
+}