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

apps/extension/src/tools/window.ts

@@ -0,0 +1,82 @@
+// Window-tool handlers: `tool.window_resize` resizes the session's
+// Agent Window via `chrome.windows.update`.
+
+import type { SessionManager } from "@/session-manager/manager";
+import type { RpcError } from "@/transport/types";
+import { validateWindowSize } from "./session";
+import { isRpcError, lookupSession } from "./shared";
+
+/**
+ * Mirror of bsk-protocol `WindowResizeParams` /
+ * `WindowResizeResult` (see crates/bsk-protocol/src/tools/window.rs).
+ */
+export interface WindowResizeParams {
+  session_id: string;
+  width: number;
+  height: number;
+}
+
+export interface WindowResizeResult {
+  window_id: number;
+  width: number;
+  height: number;
+}
+
+/**
+ * Subset of `chrome.windows` we depend on, injectable so unit tests
+ * can fake the browser without monkey-patching the global `chrome`.
+ */
+export interface WindowResizeApi {
+  update(windowId: number, updateInfo: { width: number; height: number }): Promise<unknown>;
+}
+
+export const chromeWindowResizeApi: WindowResizeApi = {
+  async update(windowId, updateInfo) {
+    return chrome.windows.update(windowId, updateInfo);
+  },
+};
+
+/**
+ * Handler for `tool.window_resize` (called by the daemon over WS).
+ *
+ * Resizes the session's Agent Window to the given outer dimensions in
+ * CSS pixels. chrome API failures are surfaced as `protocol_error`
+ * (§4.5 reserves `cdp_failed` for raw CDP errors).
+ */
+export async function handleWindowResize(
+  manager: SessionManager,
+  params: WindowResizeParams,
+  api: WindowResizeApi = chromeWindowResizeApi,
+): Promise<WindowResizeResult | RpcError> {
+  const ctxOrErr = lookupSession(manager, params, "window_resize");
+  if (isRpcError(ctxOrErr)) return ctxOrErr;
+  const ctx = ctxOrErr;
+
+  const sizeOrErr = validateWindowSize(params.width, params.height);
+  if (isRpcError(sizeOrErr)) return sizeOrErr;
+  if (!sizeOrErr) {
+    // validateWindowSize only returns undefined when both are absent,
+    // which is valid for session_start but not for an explicit resize.
+    return {
+      code: "invalid_params",
+      message: "window_resize requires width and height",
+    };
+  }
+
+  try {
+    await api.update(ctx.agentWindowId, {
+      width: sizeOrErr.width,
+      height: sizeOrErr.height,
+    });
+  } catch (err) {
+    return {
+      code: "protocol_error",
+      message: err instanceof Error ? err.message : String(err),
+    };
+  }
+  return {
+    window_id: ctx.agentWindowId,
+    width: sizeOrErr.width,
+    height: sizeOrErr.height,
+  };
+}