feat: support Agent Window sizing (session start --width/--height, window resize) (#52)

- `bsk session start --width <px> --height <px>` opens the Agent Window
  with the given outer size (chrome.windows.create)
- new `bsk window resize --session <id> --width <px> --height <px>`
  resizes the Agent Window at runtime (chrome.windows.update)
- both validated on CLI, daemon and extension sides (100..=7680 CSS px)
- protocol: new tool.window_resize method (BrowserMutation) and
  optional width/height on tool.session_start params; schemas regenerated

crates/bsk-protocol/schema/tool_session_start_params.json

@@ -12,8 +12,26 @@
         "null"
       ]
     },
+    "height": {
+      "description": "Optional Agent Window outer height in CSS pixels (100..=7680).",
+      "type": [
+        "integer",
+        "null"
+      ],
+      "format": "uint32",
+      "minimum": 0.0
+    },
     "session_id": {
       "type": "string"
+    },
+    "width": {
+      "description": "Optional Agent Window outer width in CSS pixels (100..=7680).",
+      "type": [
+        "integer",
+        "null"
+      ],
+      "format": "uint32",
+      "minimum": 0.0
     }
   }
 }