feat(session): support unfocused agent windows (--no-focus) (#87)

`bsk session start --no-focus` opens the Agent Window in the background
without stealing focus. The flag threads focused=false through
CLI -> daemon IPC -> tool.session_start -> chrome.windows.create; an
omitted `focused` keeps the extension default (true) so older clients
are unaffected.

Reworked onto current main so it composes with #52 window sizing:
AgentWindowApi.create / SessionManager.start (extension), start_session
(CLI) and the daemon's session start now take options objects/structs
({ size, focused }) instead of colliding positional params.

Co-authored-by: alectimison-maker <alec.timison@gmail.com>

apps/extension/src/session-manager/manager.ts

@@ -25,6 +25,14 @@ export interface SessionManagerOptions {
   now?: () => number;
 }
 
+/** Options for starting a session's Agent Window. */
+export interface SessionStartOptions {
+  /** Optional Agent Window outer size in CSS pixels. */
+  size?: { width: number; height: number };
+  /** Defaults to true so existing clients keep visible Agent Windows. */
+  focused?: boolean;
+}
+
 /**
  * Owner of all live agent sessions inside the extension.
  *
@@ -128,18 +136,11 @@ export class SessionManager {
    * Returns the created window id so callers can echo it back to the
    * daemon in the `tool.session_start` reply.
    */
-  async start(
-    sessionId: string,
-    size?: { width: number; height: number },
-  ): Promise<SessionContext> {
+  async start(sessionId: string, opts: SessionStartOptions = {}): Promise<SessionContext> {
     if (this.sessions.has(sessionId)) {
       throw new Error(`[bh] session ${sessionId} already exists`);
     }
-    // Only pass `size` when given so the no-size call shape (and its
-    // chrome.windows.create payload) stays exactly as before.
-    const windowId = size
-      ? await this.agentWindow.create(AGENT_WINDOW_HOME, size)
-      : await this.agentWindow.create(AGENT_WINDOW_HOME);
+    const windowId = await this.agentWindow.create(AGENT_WINDOW_HOME, opts);
     await this.agentWindow.ensureActiveTab(windowId, AGENT_WINDOW_HOME);
     const ctx: SessionContext = {
       sessionId,